ERGs primarily assess photoreceptor and bipolar cell function, but may miss differentiation defects in other cells. To test for subtle differences we stained the Rb/E2f1 DKO retina with 43 markers (Table S1). Thirty-two proteins displayed identical patterns in WT, E2f1 KO, and Rb/E2f1 DKO retina (Table S1). The other 11 markers revealed a cell-cycle– and apoptosis-independent differentiation defect in SACs. We first studied Calb2 (calretinin), which marks a subset of amacrine and ganglion cell bodies as well as three tracks corresponding to their processes in the IPL (Figure 3A). Normal Calb2 staining was seen in the E2f1 KO IPL (data not shown). However, only one Calb2+ track was evident in the Rb KO IPL, and this defect was not rescued in the Rb/E2f1 DKO retina (Figure 3A). We quantified Calb2+ cell bodies in the Rb KO INL (corresponding to amacrine cell staining only) and observed a reduction from P8 onwards (Figures 3C and S6).