The PALS1 or MPP5 protein, a membrane associated guanylate kinase (MAGUK), plays an important role in correctly localizing CRB1, CRB2, CRB3, MUPP1, and VELI3 at the subapical region (SAR) adjacent to adherens junctions (AJs) at the outer limiting membrane (OLM). Other MAGUK proteins are present in the retina as well. These are e.g. the photoreceptor specific MPP4 and the ubiquitously expressed MPP3 at the OLM. The physiological roles of MPP3, MPP4, and PALS1 have yet to be revealed. We have generated mice that lack MPP4 and showed that MPP4 is essential for correct localization of PSD95 and VELI3 at the outer plexiform layer (OPL) of the retina. We will also sort out possible overlapping and unique physiological functions, and the mechanisms where these proteins are involved in.