Early embryonic death of BUBR1-null embryos. (A) E3.5 blastocysts of the indicated genotypes were harvested, cultured in vitro, and examined at various times by phase-contrast microscopy. Data are representative of at least 3 independent experiments. Original magnification × 200. (B) At the end of the culture period, cells from the embryos shown in panel A were collected and subjected to genotyping by PCR. The leftmost lane contains DNA size markers.