1Neural Circuit Laboratories

016/j.cub.2007.04.058with thederivativesof theBarthastrainof thealphaher-pesvirus pseudorabies virus(PRV) [4, 5] that express GFP [6–12]. Bartha-strain derivatives spread only in the retrograde direction [13]. There is evidence that infected cells function normally for a while during GFP expression [7]. Here we combine transsynaptic PRV labeling, two-photon laser microscopy, and which are known targets of melanopsin-expressing gan-glion cells [22, 23]. Three and a half to four days after virus infection, 99% (79/80 cells counted from two infected retinas) of the la-beled cells were melanopsin positive, whereas the frac-tion among all ganglion cells is estimated at 1%–2 % [24, 25], suggesting that during the first wave of infection, almost all labeled ganglion cells are ipRGCs. However