A family of cytokine-inducible SH2 proteins (CISs) has recently been identified and the members are growing in number. In this family, the central SH2 domain and approximately 40 amino acids at the C-terminus (CIS homology domain; CH domain) are well conserved, while the N-terminal region shares little similarity and varies in length. Most CISs appear to be induced by several cytokines and at least three of them (CIS1, CIS3 and JAB) negatively regulate cytokine signal transduction. Forced expression of CIS1 inhibits STAT5 activation by binding of CIS1 to cytokine receptors, and CIS3 and JAB directly bind to the kinase domain of JAKs, thereby inhibiting kinase activity. Therefore, these CIS family members seem to be present in a classical negative feedback loop of cytokine signaling. They may also play a role in the mutual suppression of cytokine actions frequently found in immune and inflammatory responses. Precise molecular mechanisms of the signal inhibition and their physiological functions will be addressed in the near future. The CH domain is also found in several interesting genes containing WD-40 repeats, SPRY domains, ankyrin repeats, and GTPases. However, the function of the CH domain remains to be determined.
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