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Meet Bill Durham: Lactase Persistence Expert

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    My name is Bill Durham. I'm Bing Professor in
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    Human Biology, in human biology and anthropology at Stanford University.
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    >> Well, we're really happy you're joining us, especially to talk about a
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    very important concept. Now, we've just
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    been learning about lactose intolerance, and about
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    the idea that as all infants, we had the ability as mammals to
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    digest these lactose containing products. And as
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    we get older, our lactase expression diminishes.
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    But in some people, the lactase gene sort of stays
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    on, and we continue to make lactase. And so I guess
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    we'll start out by asking, why, in human evolution, would
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    something like this happen? How could something like this even happen?
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    >> Well from a pan-mammalian background, where the
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    lactase activity peaks around birth and then tails
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    off quickly into the weaning period and into
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    adulthood, we assume, we know that that's pan-mammalian.
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    Even your house cat is lactose malabsorbed, a malabsorber
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    or lactose intolerant. At least every house cat that's
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    been tested is, even though people will give milk
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    to their house cats. And we don't know is
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    whether the people have found the threshold, or the
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    cat's found the threshold, to avoid diearaha and other
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    consequences. What's interesting, then, is from that background of
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    a peak, some human populations show persistant lactase activity into
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    adulthood, while the majority continue to decline.
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    So it's that, minority of lactose-tolerant adults
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    that are really fascinating. it's a wonderful
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    question. Because here's a case where you have
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    an intraspecific polymorphism. You have the existence
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    in, if the ongoing persistence of a genetic
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    difference between human populations, with regard to
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    some relatively simple trait. And a trait that
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    has to be relatively recent in human history.
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    >> Sure.
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    >> There were no sources of milk prior to dairy animals.
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    >>
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    [LAUGH].
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    >> So, what we have here is, I mean,
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    of the mother's milk, there were no mammalian sources of
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    milk for human populations before agriculture. So, we assume
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    that the panamalian background, means that at the dawn of
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    history, human populations were all lactose intolerant. And that
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    there would be at best, a mutationly low frequency of
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    individuals that varied on that characteristic. And because it was
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    a very low variation and there was no milk available,
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    we would have no way of knowing how much
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    there was or how often it recurred and so on.
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    But what we have then, in the meantime, is
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    this great invention called agriculture. The domestication of plants and
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    animals at the service of the human food supply.
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    We have every reason to believe that, the main animals
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    that are used for milk today were originally domesticated for
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    meat. They were valued first for meat, and that milk
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    was, as the expression goes, was part of
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    a secondary products revolution where they realize you
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    could use more then the meat from these
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    animals. Goat, sheep, you know, cattle and so on.
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    And so they already probably had them in
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    close association with human communities. They were probably already
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    harvesting the meat regularly. They probably had mastered,
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    over winter, water supply, hearding activities, and that sort
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    of thing. And then suddenly there's evidence for using the,
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    the milk product of these animals. some of the earliest evidence comes from
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    Sahara rock paintings that are dated to 4,500 B.P., that show,
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    in fact, bringing the calf up near the
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    female, the adult female, and a human being underneath.
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    Now we infer that the human being underneath
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    is doing this. What else is a human being
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    doing under a cow?
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    [LAUGH]
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    and the question really is what was the added value of drinking fresh milk?
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    It's complicated on the one hand, because
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    you had the meat value of these animals
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    you domesticated. So, it seems unlikely that
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    it would be just the protein or food
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    supply. Yet, that was important to our
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    argument. The question, and it's also complicated because,
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    there are ways to get some food value from milk without drinking it fresh. You
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    can all societies that I know of, have ways of processing fresh milk into low
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    lactose forms. Sour milk is a low lactose form. All the things like yogurt and
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    kefir, drinks that are widely known in some
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    places, and most cheeses, break down the lactose
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    externally using lactobacillus bacteria, using a separation of curds and whey.
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    Lactose is water soluble, it drains off with the whey, and the
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    curds are free of lactose. So most cheeses have very little
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    lactose concentration, something like yogurt is
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    full of lactobacillus, that's already digesting.
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    >> Sure.
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    >> The lactose for human consumption. So here's the real challenge. In a world
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    where most populations have ways to process milk
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    and to get everything that's in the milk.
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    >> Mm-hm.
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    >> Why would some populations still evolve this persistent lactase activity?
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    >> Sure.
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    >> Because there are external sources of
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    lactase, why does it have, what, what was
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    the advantage of producing it internally to the
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    human organism? The leading argument of the day
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    is that calcium, that the advantage of drinking fresh milk, is the
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    advantage of an additional calcium supply especially in areas where there is,
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    a calcium deficiency caused either by inadequate calcium in the
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    diet. Well, that's not too likely if you're drinking milk.
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    >> Sure.
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    >> but a vitamin D scarcity. And then
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    people say, well, what's the advantage? Milk is obviously
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    rich in calcium but what's the advantage of having
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    why, why can't you again just eat the yogurt?
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    >> Mm-hm.
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    >> And why can't you again just eat the milk product, the cheese,
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    which is going to have the calcium in it? Except for the water soluble calcium
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    which you might lose some of. What's
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    interesting with some research what's called the lactose
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    effect. If you feed human infants from birth,
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    in a compare, a controlled comparison, cow's milk
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    >> Mm-hm.
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    >> from birth, or mother's milk, versus soy milk, you see, beginning in a
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    matter of, of a few days, you see a big differential in the amount of
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    bone absorbed calcium, in other words, the calcification of bone
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    >> Mm-hm.
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    >> Is enhanced, you can, this is
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    controlling for the amount of calcium, you take
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    soymilk, and you supplement the calcium in it, so that it matches the cows milk.
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    They have the same calcium, and the
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    presence of the lactose enhances the absorption of
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    the calcium according to this, these data. it's
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    a magnificent curve and it, what it shows
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    is that the ability to drink fresh milk
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    has the advantage of making the calcium that's
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    there in equal proportion in the two milks.
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    It makes it more available to the infants
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    who can absorb, who can digest the lactose.
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    So what's particularly interesting about that is it's
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    saying that a lactase persistence compensates for the
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    vitamin D deficiency that's built into the solar
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    radiation pattern on the surface of our Earth. So
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    that milk, like epidermal demelanization, they follow similar curves.
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    >> Mm-hm.
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    >> And that you get the persistent lactase
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    activity. And the lactose tolerance at high latitudes.
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    >> Right. And just to sort of reword what you say here to make
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    sure you know we're clear on this, in the case for skin pigmentation,
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    as you've sort of alluded to, the idea
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    is as human migration moved away from the equator
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    >> Yeah.
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    >> to higher and I guess we should also say lower latitudes
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    >> Yes.
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    >> They had less sunlight and so they
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    >> Well, you can say higher latitudes north and south.
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    >> Sure higher latitudes north and south. They lost pigmentation in the
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    skin, because they needed to let more sunlight in for vitamin D.
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    >> There was selection for the entrance of more sunlight.
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    It wasn't that they recognized the need.
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    >> Sure. Good, yes. Exactly
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    >> The same with milk, it wasn't that
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    we were sitting around here, wow, hoping and wishing
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    we could have another source of calcium. What's
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    happening is that those that happened to have this
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    >> Mm hm.
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    >> Out-reproduced by, you can even test whether it's a 3%, a 2%, a 5%. How
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    much does the difference have to be in
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    the long run, in terms of differential survival and
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    reproduction, in order for this to work out in
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    the time frame that we have from 4,500 years B.P.,
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    or whatever. And I'm one of those who thinks
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    that it's actually a pretty small advantage. A pretty small
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    advantage, but persistent generation after generation, and no fail,
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    it's there. If you have the lactose mechanism for absorbing
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    calcium and making strong bones, less likely to have
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    rickets, less likely to have Harrison Sulcus, less likely to
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    have broken bones in young adulthood and
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    all those things. You'll have that rigid calcification
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    of bone, which you can see starting
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    right out in infancy with the lactose affect.
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    >> Thank you so much.
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    >> Of course,
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    thank you.
Title:
Meet Bill Durham: Lactase Persistence Expert
Video Language:
English
Team:
Udacity
Project:
BIO110 - Tales from the Genome
Duration:
09:13

English subtitles

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