Adhesion molecule that promotes lamina-specific synaptic connections in the retina. Expressed in specific subsets of interneurons and retinal ganglion cells (RGCs) and promotes synaptic connectivity via homophilic interactions. Homodimer; mediates homophilic interactions to promote cell adhesion (PubMed:15703275). Interacts (via PDZ-binding motif) with MAGI1, MAGI2, DLG2, DLG3 and DLG4 (PubMed:20219992). Does not mediate homophilic interactions (PubMed:15703275). Expressed by non-overlapping subsets of retinal neurons. Sdk1 and Sdk2 are expressed in non-overlapping subsets of interneurons and retinal ganglion cells (RGCs) that form synapses in distinct inner plexiform layer (IPL) sublaminae (at protein level). Highly expressed in many fetal tissues, inlcuding kidney but shows markedly lower expression in adult organs. Expression in kidney is high throughout development with maximal expression occurring near birth. the PDZ-binding motif mediates interaction with PDZ domain-containing proteins MAGI1, MAGI2, DLG2, DLG3 and DLG4 and is required for is required for synaptic localization in photoreceptors. Ig-like C2-type domains 1 and 2 mediate homophilic interactions. Belongs to the sidekick family.