Cover legend: Triple labeling of striatal medium-sized spiny neurons with DARPP-32 (red immunofluorescence) and cocaine-induced p-ERK (blue immunofluorescence) in BAC-EGFP transgenic mice in which D₁-receptor-expressing cells are green. Neurons display a different color code depending on the presence and intensity of the three markers. Image is a laser-scanning confocal microscope single section. Picture by Jesus Bertran-Gonzalez and Emmanuel Valjent. For more information, see the article by Bertran-Gonzalez et al. in this issue (pages 5671–5685).