RB1 and teratoma: In humans, OTs are known to arise from multiple sources including, PGCs that fail to enter meiosis I during fetal development, and premature activation of fully formed oocytes following the completion of meiosis I or II [6,37] Because teratoma formation in Rb1-cKO mice was first detectable in 8 to 10-week old ovaries, sustainment of a PGC state seemed unlikely to be the underlying cause.