– Limitin, also called IFN-ζ, is a secreted interferon (IFN)-like glycoprotein. Limitin has approximately 30% sequence homology with IFN-α, IFN-β, and IFN-ω and binds to the IFN-α/β receptors. Like IFN-α and IFN-β, limitin has antiproliferative, immunomodulatory, and antiviral properties, it is unique in lacking influence on myeloid and erythroid progenitors. Similar dose requirement between limitin and IFN-α was observed for the enhancement of cytotoxic T lymphocyte activity, the augmentation of MHC class I expression, and the growth inhibition of a myelomonocytic leukemia cell line.