The human genome is spread across 23 pairs of chromosomes. During cell division, each chromosome must first duplicate, and then separate equally into the two resulting daughter cells. This process is called chromosome segregation. Unequal segregation leads to unbalanced cells with too few or too many chromosomes, which usually stop dividing, but occasionally divide in an uncontrolled manner and form tumours. Consequently, chromosome segregation is thought to be tightly controlled and highly accu rate. I have identified specific stages of normal blood cell development where many cells contain chromosomal imbalances. This suggests that problems with chromosome segregation do not always result in tumours. I plan to look for additional stages of development where many unbalanced cells are produced, and then understand why this happens at some points in development and not others. I will also study mice with mutations that increase segregation errors, to determine how different types of ce ll in the body cope with