A previously unrecognized homolog of the cytoplasmic ("nonmuscle") myosin heavy chain (MyHC) genes has been identified in public databases, constituting a distinct branch of the cytoplasmic/smooth muscle myosin family. The aim of this study was to examine if this putative gene, nonmyosin II-C, is expressed, and determine what differentiates it from other cytoplasmic myosins. We generated specific DNA probes and antibodies for MyHC II-C. RNA blot analyses showed a single band at 7.5 Kb, with highest levels in human skeletal muscle, lung, colon and heart. MyHC II-C tissue distribution patterns among a variety of organs, and among different parts of the brain and cardiovascular system, varied considerably more than those of other cytoplasmic myosins, which are more ubiguitously expressed. Furthermore, unlike other myosins, its mRNA levels in adult tissues are substantially higher than in embryonic tissues. Immunoblot analysis of mouse tissues revealed a heavy chain band at 225 kD, with highest levels in lungs and intestines. Immunofluorescence localized the protein to the apical portion of epithelial cells, and to intercalated discs and z-lines in the heart. As expected, baculovirus expression of a flag-tagged heavy meromyosin (HMM) II-C, and extraction of myosin II-C from lungs, yielded a product that binds actin, and detaches from it in presence of ATP. Interestingly myosin II-C can be induced in macrophages by treatment of these cells with sodium butyrate and trichostatin A, both inhibitors of histone deacetylase. Present experiments are directed towards generating null mice and mice containing a single amino acid mutation in a conserved region (between all myosin IIs) of the gene.