Targeted disruption of the mouse Brd1 gene. (A) Strategy for making a knockout allele for Brd1 by homologous recombination in ES cells. B, BamHI; N, NcoI; E, EcoRI; K, KpnI; S, SalI. (B) Developmental defects in Brd1^−/− embryos. Abnormal lenses with disoriented optic cups (top) and neural tube disclosure (bottom) in Brd1^−/− embryos at 12.5 dpc. Sections were stained with hematoxylin and eosin. (C) Appearance of wild-type (Brd1^+/+, left) and Brd1^−/− (right) yolk sac at 10.5 dpc. (D) Absolute numbers of total cells, c-Kit⁺ hematopoietic progenitors, and Ter119⁺ erythroid cells in 10.5 dpc yolk sac from wild-type (left bar, n = 5) and Brd1^−/− (right bar, n = 3) embryos. The results are shown as the mean ± SE *P < .05, **P < .005.