THYN1 (thymocyte nuclear protein 1) is a nuclear protein that functions as a reader of 5-hydroxymethylcytosine (5hmC), a modified cytosine base involved in epigenetic regulation. The protein contains a conserved DUF55 domain (residues 55-221) with a three-dimensional fold similar to YTH RNA-binding domains, suggesting potential involvement in RNA-related functions 1. THYN1 localizes to the nucleoplasm and fibrillar centers where it interacts with other proteins [GO annotations]. Clinically, THYN1 deletions are associated with Jacobsen syndrome (JBS), a contiguous gene deletion disorder affecting chromosome 11. Loss of THYN1, along with other genes including FLI1, ETS1, and JAM3, contributes to the immunodeficiency phenotype observed in JBS patients, characterized by reduced T and B cell populations and low immunoglobulin M levels 2. THYN1 deletions have also been identified in copy number variant studies of tetralogy of Fallot (TOF), a congenital heart disease, though the specific contribution of THYN1 to cardiac pathogenesis remains unclear 3. Recent immunologic analysis suggests THYN1 may play a role in thymic development, though ETS1 deletions appear more directly associated with abnormal recent thymic emigrants 4. Further functional studies are needed to fully elucidate THYN1's specific role in immune and developmental processes.