PAX6 and retinoblastoma: If the persistent progenitors observed in aE2f TKO mice are remnants of the retinoblastoma cell of origin, results from TEM analysis of Pax6 TKO mice support the hypothesis that the retinal cell of origin fails to develop in the absence of Pax6 as Pax6 TKO retinae show no evidence of persistent progenitors (data not shown).