LST1 (leukocyte specific transcript 1) is an MHC class III-encoded gene that functions as a transmembrane adaptor protein with immunomodulatory roles 1. Located approximately 4 kilobases upstream of the lymphotoxin beta gene in the TNF region 2, LST1 is constitutively expressed in leukocytes, dendritic cells, and monocytes, with expression inducible by interferon-gamma 3. The gene undergoes extensive alternative splicing, generating 14+ splice variants encoding both membrane-bound and soluble isoforms 4. Functionally, LST1 isoforms inhibit lymphocyte proliferation through transmembrane signaling, with the LST1/C variant showing particularly potent inhibitory effects 4. LST1 acts as a negative regulator of inflammatory responses via binding to protein tyrosine phosphatases SHP1 and SHP2, and facilitates tunneling nanotube formation 5. Disease relevance includes upregulation in rheumatoid arthritis and inflammatory bowel disease; LST1-deficient mice demonstrate resistance to dextran sodium sulfate-induced colitis 5, suggesting protective roles in limiting excessive inflammation 6. Extended MHC haplotypes incorporating LST1 associate with immune responses to rubella vaccination 7, indicating clinical significance in vaccine-induced immunity and potentially in autoimmune disease pathogenesis.