RB1 and retinoblastoma: Since ~ 40% of RB tumors arise from a germline mutation [96], there is biologicalplausibility for developmental abnormalities associated with RB haploinsufficiency.However, despite mouse studies demonstrating lethality for embryos withRb1-/- genotypes as well as neural and hematopoieticabnormalities; heterozygotes did not have any observable defects [97].