POU5F1 and teratoma: More importantly, clusters of Oct4-GFP expression-positive cells formed by culturing E8.75 hindgut in SB239063 appear after 6–8 days of culture (Figure 6B), whereas teratoma formation occurs at a high frequency in the developing testis of 129/Sv inbred mouse strains (Stevens, 1980) and Pten-deficient mice (Kimura et al., 2003), where their teratoma foci formation is observed at the E15.5.