F1 v 6.5 5–6 149 21 (14%) 0 4 4 (19%) F1 v6. 5 sc84 8–9a 46 6 (13%) 2 2 2 (33%) F1 rtTA2∆ SD-18 8–9a 32 7 (22%) 0 1 1 (14%) Total 227 34 (15%) 2 7 (21%) 7 (21%)

129 v 18.6 5–6 107 19 (19%) 1 4 0 129 J1 11 145 26 (18%) 2 4 0 129 (high) J1 35b 92 23 (25%) 10 0 0 129 LJG-13 8–9a 74 8 (11%) 4 0 0 Total 418 76 (18%) 17 8 (11%) 0 (0%)

We produced embryos by transfer of nuclei from ES cells (cultured for 1–5 d without feeder cells in standard ES media containing 1,000 U/ml leukaemia inhibitory factor (LIF), but only 5% fetal calf serum) into enucleated B6D2F1 oocytes5. We activated the oocytes by exposure to Sr++-containing media, then cultured them in vitro for 3 d before transfer into 2.5-days post-coitum (dpc) pseudopregnant Swiss females. We performed caesarean sections at 18.5–19.5 dpc for all recipient females followed by fostering of any live pups. F1 refers to the 129SvJae× C57BL/6 genetic background. Surviving cloned animals were all derived from donor F1 ES-cell nuclei because all had agouti coat colour, whereas donor oocytes came from black, non-agouti B6D2F1 females and the cloned embryos had been transferred into pseudopregnant white Swiss females. Targeted ES-cell lines are italicized. aES cells were targeted or subcloned at 4–5 passages and grown for another 4 passages before transplantation. bHigh passage. cOnly embryos which had reached morula or blastocyst stage were transferred.