Although Interferon (IFN) was first described as an antiviral agent both IFN-α2a (Roferon-A®, Hoffman-La Roche) and IFN-α2b (Intron-A®, Schering-Plough) were licensed by the U.S. FDA for the treatment of the B-cell neoplasm known as Hairy Cell Leukemia (HCL). This evidence concerns the gene IFNA2 and hairy cell leukemia.