I can't find my recent entry which indicated not much is known, but here is an entry about a coming conference:

https://www.the-scientist.com/sponsored-webinars/stem-cells--opportunities--hurdles--an...

"Since Sir. Martin Evans’ 1981 identification of embryonic stem cells in mice, stem cells have been at the center of the drive to revolutionize medicine and the drug discovery process. In 1998, human embryonic stem cells were grown in a lab, and the field was further boosted in 2006 with the pivotal discovery of induced pluripotent stem (iPS) cell techniques, which removed the need to destroy embryos. But stem cell research has not come without hurdles and controversy. How far has research come since the first groundbreaking reports were published? For further insight into ongoing challenges in the stem cell arena, the mechanisms and roadblocks encountered in iPS cell technology, and the grand opportunity stem cells represent, The Scientist is bringing together a panel of experts who will share their research, explore cellular reprogramming, and discuss the next steps.

"Topics to be covered:
Molecular mechanism of induced pluripotency
Roadblocks to iPSC reprogramming, and erasing transcriptional memory in cellular reprogramming"

Comment: all that is known is from very recent work