a Reintroduction of 129/Sv genes by backcross of EGT-315 B6 (F9-F11) (mean = 0.79, SD = 2.23, n = 40) with 129/Sv increases ERV-GFP expression in EGT-315 B6 x 129/Sv (F1) mice (mean = 8.91, SD = 14.0, n = 32). C57BL/6 (mean = 0, SD = 0, n = 9) and (129/Sv x C57BL/6) F1 wt littermates (mean = 0.063, SD = 0.122, n = 9). Red bars show mean. Flowcytometry of blood leukocytes at 3 weeks of age. ***p = 0.0008. Ordinary one-way ANOVA Tukey´s multiple comparsion test. b Epigenetic DNA methylation status of a 549 bp DNA fragment of ERV-GFP in GFP positive (+) vs GFP negative (-) cells isolated from spleen cells of EGT-315 mice. DNA was digested with methylation sensitive HpaII (recognition site CCGG) and normalized with Plcg2 specific PCR w/o HpaII recognition site. ES-cells are ERV-GFP containing but GFP-expression negative and ERV-GFP infected WEHI-231 cells strongly express GFP. From individual mice sorted GFP+ cells (mean = 4.91, SD = 4.32, n = 9), sorted GFP- cells (mean = 3.3, SD = 1.9, n = 15), WEHI-231 GFP+ (mean = 0.37, SD = 0.12, n = 4), ES cells GFP- (mean = 26.9, SD = 8.3, n = 4). Sorted GFP+ cells vs Sorted GFP- cells nsP = 0.752. Sorted GFP+ cells vs ES cells GFP- ****P < 0.0001. Sorted GFP- cells vs ES cells GFP- ****P < 0.0001. WEHI-231 GFP+ vs ES cells GFP-****P < 0.0001. Ordinary one-way ANOVA Tukey´s multiple comparsion test. Summary of 3 experiments. c Q-PCR of RNA expression of the GFP and Env region (549 bp) of ERV-GFP transcribed in spleen of EGT-315 mice (left panel). Sequence analysis of RNA expression analysis by RT-PCR-transcripts of EGT-315 mice with and without human Apobec3 (hA3Tg). Each dot depicts the mean number of mutations of 4–8 clones sequenced per individual mouse of the different genotypes (right panel). EGT-315 B6 (green bar, mean = 1, SD = 1.9, n = 4), EGT-315 Tlr3−/−Tlr7−/−Tlr9−/− (blue bar, mean = 0.06, SD = 0.09, n = 5), EGT-315 Tlr3−/−Tlr7−/−Tlr9−/− hA3 (purple bar, mean = 7.3, SD = 6.0, n = 5). Statistics of mutations EGT-315 Tlr3−/−Tlr7−/−Tlr9−/− versus EGT-315 Tlr3−/−Tlr7−/−Tlr9−/− hA3 *p = 0.028 by Ordinary one-way ANOVA Tukey´s multiple comparsion test. Source data are provided as a Source Data file.