Spontaneous genetic variation has been described in cultured cells almost from the beginning of genetic analysis in cell culture. In one example, published in 1968, Atkins and Gartler proved that the mutation rate in culture is the same whether selection is applied or not (Atkins and Gartler, 1968). However, there has been relatively little report of spontaneous reversion in iPSC created from genetic variant cells, except for cases in which naturally occurring reversion of cultured primary cells have been reprogrammed into iPSC (Tolar et al., 2014, Maclean et al., 2012). Careful studies demonstrate that much of the variation between iPSC lines may be attributed to genetic variation in source skin fibroblasts (Abyzov et al., 2012) and that skin cells accumulate mutations in situ (Martincorena et al., 2015). Our results show that cultured iPSC have the capacity to undergo genetic rearrangement, in this case a reversion from ATM−/− to ATM+/−. All available evidence, including at least the 1.4 kb spanning the c.217_218 delGA position to the location of rs2066734, supports the conclusion that gene conversion of ATM occurred, replacing the maternal
