JAK3 (Janus kinase 3) is a non-receptor tyrosine kinase that plays essential roles in immune system development and function through cytokine signaling pathways 1. JAK3 mediates signaling downstream of cytokine receptors that share the common gamma chain, including IL-2R, IL-4R, IL-7R, and IL-15R 1. Upon cytokine binding, JAK3 becomes activated and phosphorylates STAT proteins (particularly STAT3, STAT5, and STAT6), which then dimerize and translocate to the nucleus to regulate gene transcription 2. The JAK3 gene is located on chromosome 19.1 and consists of 19 exons 3. Mutations in JAK3 cause severe combined immunodeficiency (SCID) characterized by absent T cells and NK cells but normal B cell numbers (T-B+NK- phenotype) 4. This complete developmental block in T cell maturation can be corrected through gene targeting approaches 4. JAK3 has emerged as a therapeutic target, with JAK3 inhibitors showing promise in immunosuppression for organ transplantation and in treating steroid-resistant asthma by suppressing ILC2 cells 56. Specific JAK3 mutations can also influence cancer therapy responses by modulating PD-L1 expression through the STAT3 pathway 2.