In order to determine whether the teratoma-initiating potential in the differentiated population was due to an increased number of failed to differentiate cells in vitro as shown for transformed iPS cells [3], we performed flow cytometry for Oct4 and SSEA1 on day 4 of differentiation, and found no statistically significant difference in the percentage of SSEA1 positive cells or SSEA1/Oct4 double positive cells compared to wild type (Fig. 3A). Here, POU5F1 is linked to teratoma.