Constitutive activation of NOTCH1 signaling in Sertoli cells causes gonocyte exit from quiescence

► Using Notch reporter mice, we show that Notch signaling is active in Sertoli cells. ► Mutant mice constitutively over-expressing NICD1 in Sertoli cells are sterile. ► Gonocytes start dying at E15.5 due to premature differentiation. ► Factors required for proper gonocyte maintenance are dysregulated in the mutant. ► Excess or improperly-timed Notch activation impairs fetal gonocyte maintenance.