Role of the mouse VNO in gender discrimination. (A) Behavioral analysis of TRP2 -/- males reveals that non-VNOrelated sensory cues such as olfactory, auditory, tactile, or visual cues trigger mating behavior with other mice irrespective of their gender. Thus, in the absence of VNO activity, mating is the default behavior of the male encountering a conspecific. (B) In a wild type male, gender identification signaling is ensured by VNO activity. The detection of male pheromones by a male counters the default behavior initiated by non-VNO mediated sensory inputs, resulting in inhibition of mating and initiation of aggressive behaviors. (C) In contrast, identification of a female is permissive for the default behavior initiated by non-VNO mediated sensory inputs resulting in mating while inhibiting aggression.