Currently, the only way to ensure  that teratoma will not develop after human embryonic stem cell transplantation is to differentiate them in desired and mature cell type  before injection and screen them for the presence of undifferentiated cells. When such procedures  were rigorously followed, teratomas were not observed in over 200 animals transplanted with human embryonic stem cell-derived cardiomyocytes. However, unwanted and uncontrolled differentiation of human embryonic stem cells was still noticed despite following up of this procedure. Primitive population of  nestin + neuroepithelial cells, that continued to proliferate in striatum, was noticed in rats with Parkinson disease, 70 days after transplantation  of human embryonic stem cell-derived dopamine neurons. This raises a cautionary flag and suggests that even committed progenitors can proliferate excessively after transplantation, a problem that may be solved by improving purification methods.