Direct activation by expression of a phosphomimetic Dbl3 mutant that circumvents the requirement for MerTK rescues phagocytosis in induced pluripotent stem cell (iPSC)–derived retinitis pigmentosa RPE cells, indicating that the principal function of MerTK in phagocytosis is activation of the MRCKβ pathway. This evidence concerns the gene CDC42BPB and retinitis pigmentosa.