Tyrosine kinase 2 (TYK2) is a non-receptor tyrosine-protein kinase, an enzyme that in humans is encoded by the TYK2 gene. TYK2, together with three other family subtypes, namely, Jak1, Jak2, and Jak3, belong to the JAK family of tyrosine kinases that are involved in chromosomal translocation-induced fusion proteins found in anaplastic large cell lymphomas (ALCL) that lack rearrangements activating the anaplastic lymphoma kinase (ALK). TYK2 functions downstream of multiple cytokine receptors. Genome-wide association studies have linked an SNP (rs34536443) within TYK2 encoding a Proline to Alanine substitution at amino acid 1104, to protection from multiple autoimmune diseases including systemic lupus erythematosus (SLE) and multiple sclerosis (MS). TYK2 has been identified as a crucial protein in signal transduction initiated by interferons or interleukins in mammals.