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A. Childhood leukemia mutation may form in the womb

Narrator: This is Science Today. A genetic mutation linked to the most common form of childhood leukemia appears to form in the womb. Joseph Wiemels, an epidemiologist at the University of California, San Francisco, says there seems to be at least two genetic changes that cause acute lymphoblastic leukemia - one that occurs before birth and another, during childhood.

Wiemels: The first thing we'd like to know is, is this very common or does every individual who gets the mutation before birth get leukemia? And what our research suggests is that no, they don't necessarily get leukemia, they have to have the second event - the deletion of the other chromosome.

Narrator: Studies suggest this second mutation may result from an abnormal response to common, childhood infections, whereas the initial mutation seems to be the result of a developmental accident, not exposure to an environmental mutagen.

Wiemels: Really what we're trying to do is define the natural history of leukemic clone. Find out when it starts and what happens to it all up to that point. By being able to trace when these genetic events happen, we can then devise preventative measures.

Narrator: For Science Today, I'm Larissa Branin.