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Despite advances in medicine,

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cancer remains one of the most frightening[br]diagnoses patients can receive.

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What makes it so difficult to cure[br]is that it's not one illness,

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but a family of over 100 diseases[br]occurring in different types of cells.

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And one type of cancer [br]has the unfortunate distinction

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of afflicting children [br]more than any other type.

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This is leukemia,

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a cancer that begins in stem cells[br]found in the bone marrow.

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A stem cell is a bit like an infant,

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undeveloped but possessing [br]great potential.

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Many stem cells specialize[br]and become cells of organs,

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like the liver, brain and heart.

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But in some tissues,

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they can continue to divide into new [br]stem cells throughout development,

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and into adulthood in order to frequently[br]generate new cells

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and keep up with the body's needs.

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One example is the bone marrow,

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where stem cells differentiate[br]into many types of blood cells.

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That includes red blood cells,

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which carry oxygen [br]from the lungs to all tissues,

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platelets, which help stop bleeding[br]by sticking to damaged blood vessels,

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and white blood cells,

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which patrol the body,[br]destroying potentially harmful invaders.

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Every once in a while,

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something goes wrong during [br]a stem cell's specialization process

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and harmful mutations [br]occur in the cell's DNA.

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Cells with compromised DNA[br]are supposed to self-destruct,

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but some damaged cells ignore this order,

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replicating uncontrollably,[br]even as they lose their original function.

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These are what we know as cancer cells.

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It is not yet clear why leukemia[br]is the most common childhood cancer,

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but one contributing factor may be

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that leukemias are often caused[br]by just one or two DNA modifications,

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while most cancers require many of them,

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allowing leukemias to arise faster[br]than other types of cancer.

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Moreover, some DNA alterations[br]can occur in white blood cells

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during fetal development,

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further increasing the risk [br]of early leukemia.

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But though it affects more children[br]than any other cancer,

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adults constitute the majority[br]of leukemia patients overall.

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Once leukemia strikes, the damaged cells[br]reproduce in the blood and the bone marrow

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until they take up [br]all available space and resources.

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When the bone marrow [br]can no longer produce

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the required amount of functional cells,

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the blood becomes depleted.

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The lack of red blood cells

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means that muscles [br]don't get enough oxygen,

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the reduced number of platelets[br]is not sufficient to repair wounds,

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and the dearth of functional white[br]blood cells impairs the immune system,

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increasing the risk of infections.

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To restore the normal [br]function of the blood,

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leukemic cells have to be eliminated.

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But because leukemias [br]are not solid tumors,

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they can't be removed surgically.

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Instead, the cells are killed[br]inside the body using various treatments

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that include chemotherapy,

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a combination of drugs that destroys[br]quickly multiplying cells.

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Unfortunately, this has the side effect[br]of killing healthy cells,

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such as those found in hair follicles[br]or intestines.

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And in some cases, the dosage [br]required is so high

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that it kills all cells [br]in the bone marrow,

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including stem cells.

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When this happens, the body is no longer[br]able to create new blood cells on its own.

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Fortunately, outside help can come[br]in the form of stem cells

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from the bone marrow of a donor.

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Once transplanted into the patient,

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they rapidly repopulate [br]the bone marrow and the blood.

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However, bone marrow transplants[br]are a complicated process

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requiring antigen compatibility[br]between the donor and recipient

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to keep the transplanted cells from[br]from attacking the patient's own cells

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as foreign bodies.

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Unlike with blood transplants,

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there are thousands of HLA types,

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and even siblings and close relatives[br]may not have compatible bone marrow.

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If this is the case, the search [br]is expanded to a database

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containing the genetic makeup of millions[br]of voluntary bone marrow donors.

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The more potential donors there are,

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the more patients lives can be saved[br]through successful transplants.

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Leukemia may be a frightening disease,[br]but there is strength and hope in numbers.