Rudolf Jaenisch, from the Whitehead Institute for Biomedical Research, reported on iterative chemical screening to evaluate alternative culture conditions for naive human pluripotency, ultimately yielding an improved combination of five kinase inhibitors (5i/L/FA) that induces and maintains OCT4 distal enhancer activity when applied directly to conventional hESCs (Theunissen et al., 2014). Using these optimized conditions, his group demonstrated direct conversion of primed to naive ESCs in the absence of transgenes and isolation of novel hESCs from human blastocysts. They noted, however, that naive hESCs showed upregulated XIST and evidence of X inactivation, raising the possibility that X inactivation in naive stem cells in mouse and human may be different. Critically, transplantation of GFP-positive human naive hESCs into mouse blastocysts yielded no GFP-positive E10.5 embryos, either by their original method (n = 860 embryos) or by published methods (Gafni et al., 2013) (n = 436+ embryos). PCR for human mitochondria is a more sensitive assay, identifying even the presence of 1/10,000 human cells, but this also failed to detect mouse-human chimerism. Although the generation of interspecies chimeras by injection of human
