Interferon-alpha (IFN-alpha), also known as leukocyte interferon, represents a group of related but distinct proteins that share over 95% amino acid sequence homology. They are members of the type I interferon family which share a common cell surface receptor composed of two subunits, a 100 kDa ligand-binding subunit (IFN-alpha R2) and a 125 kDa ligand binding and signal transduction subunit (IFN-alpha R1) that is involved both in ligand binding and signal transduction. IFN-alpha has both anti-viral and immunomodulatory activities on target cells.