The interferons (IFNs), first described in 1957, are a group of at least five natural human glycoproteins (alpha, beta, gamma, omega and, tau) with marked antiviral, antiproliferative, and immunomodulatory activity. Only the first three types are currently used clinically. Interferon-α (IFN-α) is an heterogeneous family of more than 20 subspecies of proteins and glycoproteins. Natural lymphoblastoid IFN-α (IFN α-N1) is a mixture of nine IFN subtypes produced from a human B lymphoblastoid cell line, whereas recombinant IFN-α (IFN alfa-2a and IFN alfa-b) is produced by DNA techniques using a strain of Escherichia coli. More recently, researchers developed a so-called consensus IFN-α (CIFN) by assigning the most frequently observed amino acids at each position to generate a consensus molecule (Keeffe and Hollinger 1997). Consensus IFN-α was later produced in a recombinant system. Interferon-β (IFN-β) has antiviral and antiproliferative properties very similar to those of IFN-α and is being used as a natural fibroblast or in recombinant preparations. Two recombinant forms of IFN-β (i.e., IFN beta-1b and, more recently, IFN beta-la), have been approved for the treatment of multiple sclerosis. Interferon-γ (IFN-γ) is produced essentially by activated T lymphocytes and shares no similarities with the other IFNs. Interferon-γ primarily exerts immunoregulatory properties and is available only as a recombinant form for the treatment of chronic granulomatous disease.