... low and far from routine. Nevertheless, reprogramming with defined factors, Oct4, Sox2, Klf4 and c-Myc, is not a random event. Cells positive for SSEA-1, a marker of undifferentiated mouse ES cells (ESC), appear from cells which have lost the fibroblast marker Thy-1, prior to acquiring other pluripotent markers, e.g. Oct4, Nanog. Similarly, TRA-1-60 positive fully reprogrammed human iPSCs appear ...