Abstract A 2199-bp complementary DNA (cDNA) that encodes protein kinase C-ζ (PKC-ζ) has been isolated from mouse brain by a combination of reverse transcription and primer extension. The predicted PKC-ζ protein consists of 592 amino acids which are 99% identical to those of rat PKC-ζ. Northern blots that were probed with this cDNA revealed abundant 2200-nucleotide (nt) and 4200-nt PKC-ζ mRNAs in mouse brain in roughly equal amounts. PKC-ζ mRNA was also abundant in normal lung, kidney, and testes, and in several hemopoietic tumor lines. In all other mouse tissues and cell lines that were examined, at least faint levels of PKC-ζ mRNAs could also be detected. In tissues other than brain, the amount of PKC-ζ mRNA was less, and the smaller species generally predominated. Furthermore, in these tissues, both PKC-ζ mRNAs appear to be approximately 200 nt longer than the two mRNAs found in the brain. When the cDNA is expressed in insect cells via a baculovirus expression vector, a 75-kDa protein is synthesized which, unlike other PKC isoforms, does not bind phorbol ester, even at very high concentrations.