Cover picture: Many neural progenitor cells (NPCs) in the normal embryonic mouse brain harbor chromosome segregation defects. Bipolar NPC mitoses (top left, top center, and top right) ensure equal segregation of chromosomes. Deviations from bipolar mitosis, such as tripolar mitosis (bottom left), mitosis that harbors lagging chromosomes (bottom center), and nondisjunction of chromosomes (bottom right), frequently lead to unequal choromsome segregation and aneuploidy. An aneuploid NPC (center) that has lost one copy of chromosome 19 is revealed by a multicolor chromosome painting technique called spectral karyotyping. For details, see the article by Yang et al. in this issue (pages 10454–10460).