SDK2 (sidekick cell adhesion molecule 2) is an immunoglobulin superfamily transmembrane adhesion molecule that evolved in vertebrates through gene duplication from a Drosophila precursor 1. Its primary function involves promoting lamina-specific synaptic connections in the retina by facilitating homophilic binding between retinal ganglion cells (W3B-RGCs) and excitatory amacrine cells (VG3-ACs), connections essential for motion detection circuits 1. SDK2 operates through specific homophilic interactions mediated by its ectodomain and intracellular association with PDZ scaffold proteins 1. Beyond retinal development, SDK2 has been associated with multiple neurodevelopmental and psychiatric disorders. Genome-wide studies link SDK2 variants to bipolar disorder age-of-onset 2, depressive episodes with psychotic symptoms and childhood trauma 3, panic disorder 4, and Hashimoto's thyroiditis susceptibility 5. Additionally, SDK2 expression alterations were identified in hidradenitis suppurativa with gender-specific patterns 6 and in group 3 medulloblastoma as a super-enhancer-driven tumor-dependent gene 7. SDK2 was also identified as a novel association in idiopathic inflammatory myopathies 8. These findings suggest SDK2's pleiotropic involvement in neural circuit development and immune-related conditions, though additional functional validation is necessary to clarify disease mechanisms.