Table 2.

Embryonic lethality in transgenic animals on pure a 129/Sv background.

Line (copy number)	MF1 female (Transgenic/Total number)	129/Sv female (Transgenic/Total number)
5D3/129 (1)	13/23	1/35*
Generation 3 5D3	n.t.	1/33**
5A4/129 (2)	3/9	0/16
5A2/129 (2)	2/5	0/10
5B6/129 (4)	6/10	0/20
10–15/129 (3)	N.d.	48/109***

The transgene was present at the expected frequency (50% of the adult offspring, P = 0.5) when chimeras were mated with MF1 females but there was a significant reduction in survival of transgenics for single copy line 5D3 (number of transgenic animals/total number genotyped = 1/35, *P = 0.0001) and no survivors for the other lines when chimaeras were crossed with inbred 129/Sv females. F1 males from line 5D3 (50%:50%; MF1 × 129/Sv) were backcrossed onto 129/Sv for three generations (G3). At which point significant lethality was again noted (1/33 transgenics from eight litters, **P = 0.06). The modified transgene (10–15) bred six generations into the 129/Sv background showed no significant difference in survival (***P = 0.20).