Contrary to its role in somatic stem cells where inactivation of RB family proteins often results in stem cell expansion, increased apoptosis, altered cell fate/differentiation defects, and initiation of cancer (60), pRB-deficient male GSCs have been reported to lose their capacity to self-renew, possibly explaining why no testicular tumors were observed (52, 54). The gene discussed is RB1; the disease is cancer.