Carl Zimmer has a very nice post that examines research on an evolutionary event in the human lineage, the loss of a sugar that is ubiquitous in other mammals. The research points to loss of this sugar to an inserted Alu repeat between 1.6 and 3.8 million years ago. The link to stem cell research is that current embryonic stem cell lines cultured on media taken from other mammals. Other mammals have the sugar humans have lost, and the stem cells incorporate it into their cell membranes. So if these stem cells were inserted into a human host, the host’s immune system would be likely to tag them as foreign tissue and react to them, obviating one advantage that stem cells were supposed to provide for human medicine.
The kicker comes in that governmental policy dictates that researchers shall not fix the embryonic stem cell lines by starting over with a growth medium improved by our newfound knowledge; they either use the lines as they are, alien sugar and all, or find something else to do.
Please go give Carl’s post a read; it is well worth your time.