Adhesion molecule that promotes lamina-specific synaptic connections in the retina (PubMed:12230981, PubMed:18216854). Expressed in specific subsets of interneurons and retinal ganglion cells (RGCs) and promotes synaptic connectivity via homophilic interactions (PubMed:12230981, PubMed:18216854). Homodimer; mediates homophilic interactions to promote cell adhesion. Expressed by non-overlapping subsets of retinal neurons (PubMed:12230981). SDK1, SDK2, DSCAM and DSCAML1 are expressed in non-overlapping subsets of interneurons and retinal ganglion cells (RGCs) that form synapses in distinct inner plexiform layer (IPL) sublaminae (PubMed:18216854). Belongs to the sidekick family.