Signal transducer and activator of transcription 3 (STAT3) is a member of the STAT protein family. In response to cytokines and growth factors, STAT3 is phosphorylated by receptor-associated Janus kinases (JAK), form homo- or heterodimers, and translocate to the cell nucleus where they act as transcription activators. Specifically, STAT3 becomes activated after phosphorylation of tyrosine 705 in response to such ligands as interferons, epidermal growth factor (EGF), Interleukin (IL-)5 and IL-6.

Loss-of-function mutations in the STAT3 gene result in Hyperimmunoglobulin E syndrome, associated with recurrent infections as well as disordered bone and tooth development.Gain-of-function mutations in the STAT3 gene have been reported to cause multi-organ early onset auto-immune diseases; such as thyroid disease, diabetes, intestinal inflammation, and low blood counts, while constitutive STAT3 activation is associated with various human cancers and commonly suggests poor prognosis. In addition, two recent studies performed in APC mutant mice showed that STAT3 has an inhibiting role in colon carcinogenesis depending on tumor stage.