Olsen and Uhler (1991) isolated clones encoding human PKIA. The deduced 75-amino acid PKIA protein shares 100% and 97% sequence identity with the rabbit and mouse homologs, respectively. Northern blot analysis detected major 4-kb and minor 2-kb PKIA transcripts in skeletal muscle. Using kinase assays with transfected COS cells, Olsen and Uhler (1991) verified that the PKIA cDNA produces a heat-stable inhibitor of protein kinase. Protein extracts inhibited both the alpha and beta isoforms of the protein kinase catalytic subunit with equal efficacy. Using a transcriptional activation system, Olsen and Uhler (1991) demonstrated that elimination of a conserved alternative translation start site in PKI increased the inhibitory activity of the PKI expression vector.