(A) Horizontal retinal sections of the indicated ages and genotypes were stained for nuclei (DAPI, blue), mitosis marker PH3 (green), and Slc18a3 (red), which marks SAC soma at early stages and processes from ~P5 onwards. Arrows show mitotic PH3+ nuclei in Rb KO, Rb/E2f2 DKO, and Rb/E2f3 DKO retinas. E2f1 loss rescues the ectopic mitosis and cell death defects, but not the SAC defect. E2f2 loss has no effect. E2f3 loss does not rescue the ectopic mitosis and cell loss defects, but rescues the SAC defect. Inactivating E2f1 and E2f3 together rescues the ectopic mitosis, cell death, and SAC defects.