The Protein Kinase C (PKC) family of homologous serine/threonine protein kinases is involved in a number of processes such as growth, differentiation, and cytokine secretion. At least eleven isozymes have been described. These proteins are products of multiple genes and alternative splicing. PKC consists of a single polypeptide chain containing four conserved regions (C) and five variable regions (V). The N-terminal half containing C1, C2, V1, and V2 constitutes the regulatory domain and interacts with PKC activators Ca2], phospholipid, diacylglycerol, or phorbol ester. However, the novel PKC (nPKC) subfamily members (δ,η, η, and θ isoforms) and the atypical PKC (aPKC) subfamily members (ζ , ι , and λ isoforms) are Ca[2+] independent and lack the C2 domain. The PKC pathway represents a major signal transduction system that is activated following ligand-stimulation of transmembrane receptors by hormones, neurotransmitters and growth factors. Expression of the 90 kDa PKCα is induced by interferon-α. Generally, PKCα is expressed at very low levels in normal murine tissues, except brain. Overexpression of PKCα leads to increased growth rates and higher cell densities in monolayer cultures. A high level of expression is seen in several hematopoietic cell lines and tumors. This suggests a possible role for PKCα in tumorigenesis.