Sexual dimorphism in serum IFN-γ release following in vivo challenge with α-GalCer is abolished in ER-α–deficient mice. Ten-week-old male (□) and female (▪) ER-α^-/- mice received a single injection of α-GalCer and were killed 90 minutes later. Serum IFN-γ and IL-4 concentrations were determined by ELISA and expressed as means ± SEMs (4-6 mice per group from 2 separate experiments). In control mice that received an identical volume of vehicle solution alone, both cytokines were not detectable (< 20 pg/mL). ^**P < .01. NS indicates not significant.