The signal transducer and activator of transcription protein family consist of seven members: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6. Each protein is composed of 5 domains: : N-terminal, ‘coiled-coil’, DNA-binding, SH2 and C-terminal transactivation domain. Protein activity is dependent on phosphorylation of: tyrosine residue located in transactivation domain, which allows STAT protein to for dimers or more complexed structures and serine residue which enhances transcriptional activity of protein.

STAT proteins are involved in fundamental cellular processes such as cell growth, proliferation and apoptosis, mediation in IFN signalling, embryonic development, inflammatory and allergic immune responses. Deregulation of these proteins activation contributes to multiple disease phenotypes development.

Abnormal STAT signalling pathway activation is present in many human diseases. Lack of STAT1 block responsiveness to interferons, leading mice to suffer to bacterial and viral infections. It also causes malignant cells to undergo rapid growth and leads to increased tumor formation. Recent studies suggest a more complex role of STAT1 in carcinogenesis as well as in atherosclerosis. Experiments with mice lacking STAT2 showed defective antiviral responses to IFNα and IFNβ. What is more STAT2 activity may partly contribute to carcinogenesis. Constitutive activation of STAT3 is present in many cancers – breast, melanoma, prostate, head and neck squamous cell carcinoma (HNSCC), multiple myeloma, pancreatic, ovarian and brain tumors. STAT3 hyper activity is also observed in chronic inflammatory diseases, like rheumatoid arthritis and psoriasis. STAT4 activation is mainly linked with autoimmune diseases, e.g. Experimental Autoimmune Encephalomyelitis, an animal model of multiple sclerosis and systemic lupus erythematosus. Constitutive STAT5A and STAT5B activation is noticeable in chronic myelogenous leukaemia (CML), breast, uterine and prostate cancer and also HNSCC. In this respect, STAT5 and STAT3 share a similar function in promoting cancer and their activation has been described in 70% of solid and haematological tumors. Finally, STAT6 activation is recognized in lung and airway diseases (airway hyperactivity, mucus production), and also in regulation of allergic skin inflammation.