a, Animals expressing Cre-dependent mCherry virus in VMHvl fail to show aggression during photostimulation. Representative raster plot showing episodes of close investigation (CI; yellow ticks), mounting (green ticks) or attack (red ticks) in an mCherry-expressing Esr1cre/+ male. No attacks are evoked towards either a castrated male (upper plot) or an intact unreceptive female (lower plot) during photostimulation trials (blue bars; 473 nm, 20 ms pulses, 20 Hz, 30 s; numbers indicate mW mm−2). b, c, Activation of the non-Esr1-expressing subpopulations of VMHvl neurons is insufficient to evoke aggression. Representative raster plots illustrating photostimulation-evoked behavioural responses towards a castrated male by a wild-type (b) or an Esr1cre/+ (c) mouse injected with the ‘Cre-out’ AAV2 containing a floxed ChR2 coding sequence (Fig. 2r). Attack (red; 3.2-6.8 mW mm−2) was elicited during photostimulation trials (blue bars) in wild-type males, indicating that the floxed ChR2 construct is effective in the absence of Cre, whereas no behaviour was evoked in Esr1cre/+ males where ChR2 is expressed in Esr1−, but not in Esr1+, neurons.