Table 1. Heterozygosity for Nipbl causes postnatal lethality.

Paternal genotype	Surviving≥3 postnatal weeks			Viable at E17.5–E18.5			Resorption at E17.5
	+/+	+/−	ratio	+/+	+/−	ratio	+/+	+/−
Chimera	91	22	4.1∶1*	nd	nd	nd	nd	nd
N0 Nipbl+/−	190	40	4.8∶1*	37	30	1.2∶1†	0	2
N1 Nipbl+/−	247	54	4.6∶1*	nd	nd	nd	nd	nd

Data are presented on the frequencies of genotypes resulting from crosses between heterozygous mutant males and CD-1 females. The males referred to in the first row were the chimeras produced by injection of Nipbl+/− ES cells into C57BL/6 blastocysts, so in this case only progeny descended from ES cells (as distinguished by chinchilla coat color) were scored. Surviving Nipbl+/− mice from these crosses are referred to as the N₀ generation; their offspring with CD-1 females are referred to as the N₁ generation; their offspring with CD-1 females as the N₂ generation; and so on. Note that although the ratio of mutant to wildtype animals at E17.5–E18.5 is not significantly different from 1∶1, the presence of identifiably-mutant, but not wildtype, resorbed embryos at this stage suggests that there may be a small amount of late embryonic loss.

*: P<0.001 by chi-squared analysis when compared with Mendelian expectations.

†: P = 0.67 compared with Mendelian expectations, and P<0.005 when compared with the postnatal distribution.