Flt-1^−/− mutant endothelial cells from developing vessels display aneuploidy and chromosome aberrations. Endothelial cells from WT and flt-1^−/− ES cell–derived vessels were isolated via magnetic bead isolation and analyzed for chromosome number and abnormalities. (A-B) Giemsa-stained WT (A) and flt-1^−/− (B) endothelial cell metaphase spreads with 40 and 47 chromosomes (CS), respectively (Mus musculus 2n = 40). Arrows in panel B point to abnormal triradial chromosome configurations. (C) Scatter plot showing chromosome number in WT versus flt-1^−/− endothelial cells from developing vessels. Each dot represents one cell; red dots, cells with 40 CS; and blue dots, aneuploid cells. WT, n = 25; flt-1^−/−, n = 25. *P ≤ .05.