Mice lacking both Bim and Puma were generated by intercrossing double heterozygotes (bim+/–/puma+/–) and subsequently by intercrossing bim+/–/puma–/– males with double heterozygous females. Consistent with the previously reported partial embryonic lethality of bim–/– mice in intercrosses of bim+/– heterozygotes on the mixed 129SVxC57BL/6 background (6), only 44% of bim–/– and 40% of the expected numbers of bim–/–/puma–/– mice were born alive on the inbred C57BL/6 background. This indicates that a substantial percentage of bim–/– and bim–/–/puma–/– embryos die in utero and suggests that Bim and Puma do not have overlapping functions during embryogenesis. Additional bim–/–/puma–/– mice were generated by crossing bim–/–/puma–/– males with bim+/–/puma–/– females.