SDK1 (sidekick cell adhesion molecule 1) is a large immunoglobulin superfamily transmembrane protein that functions primarily as an adhesion molecule promoting homophilic cell-cell interactions and synaptic specificity in neural tissues 1. In the retina, SDK1 mediates lamina-specific synaptic connections through homophilic binding of its ectodomains and intracellular association with PDZ scaffold proteins, with expression in specific subsets of interneurons and retinal ganglion cells determining synaptic connectivity patterns 1. Beyond its established retinal function, emerging evidence suggests broader roles in human disease. SDK1 expression appears protective against chr7 obstructive pulmonary disease (COPD), with DNA methylation at intronic CpG sites regulating gene expression and disease susceptibility 2. The gene shows differential methylation patterns in neuropsychiatric conditions, being commonly altered in both Alzheimer's disease and major depressive disorder 3. Additionally, SDK1 polymorphisms have been associated with hypertension susceptibility in Japanese populations, with specific variants conferring protection against the condition 4. The large size of the SDK1 gene makes it susceptible to mutations and rearrangements, potentially contributing to various neurodevelopmental disorders, psychiatric conditions, and certain cancers 1.