POU5F1 and teratoma: However, more evidence is needed to conclude that accidental integration of extrinsic POU5F1 was the sole reason for teratoma formation, and we cannot deny the possibility that at least one mutation in POU5F1B in 15M38 derivatives and genetic instability (carrying the maximum copy number of more than 3 at 14q32.33 or 17q12) could cooperatively contribute to teratoma formation.