UBASH3A (ubiquitin-associated and SH3 domain-containing protein A) is a lymphoid-enriched regulatory protein located on chromosome 21.3 that functions as a negative regulator of T-cell receptor signaling and receptor tyrosine kinase degradation 1. The protein contains both ubiquitin-associated (UBA) and SH3 domains, enabling protein-protein interactions 1. Mechanistically, UBASH3A interferes with CBL-mediated downregulation of activated receptors including T-cell receptors and EGFR, promoting their surface accumulation and sustained signaling. It may also inhibit dynamin-dependent endocytosis through SH3-mediated sequestration [UniProt annotation]. UBASH3A expression is highest in spleen, peripheral blood leukocytes, and bone marrow, with notable expression during kidney nephrogenesis 12. Genetically, UBASH3A polymorphisms (particularly rs1893592 and rs3788013) associate with multiple autoimmune diseases including rheumatoid arthritis 3, systemic lupus erythematosus 4, atopic dermatitis 5, and primary sclerosing cholangitis 6. Dysregulation involves both genetic variation and epigenetic mechanisms: promoter methylation negatively correlates with UBASH3A expression and T-lymphocyte development, with higher methylation conferring reduced sepsis susceptibility 7. These findings suggest UBASH3A dysfunction contributes to autoimmune and inflammatory pathogenesis.