A mouse embryonic cell line derived from the F123-CASTXs129 F1 hybrid line engineered with both alleles of the Sox2 gene tagged - CAST allele with eGFP and s129 allele with mCherry and an suicidal fusion gene Tg(CAG-HyTK) flanked by Flippase recognition sites FRT/F3 inserted between Sox2 and it's downstream Super Enhancer to allow insertion of DNA cassettes followed by selection for integration using the RMCE strategy.