Expression of the Protein Product of the PCPH Proto-oncogene in Human Tumor Cell Lines. Exposure of Syrian hamster embryo fibroblasts to chemical carcinogens resulted in the oncogenic activation of the PCPH proto-oncogene by induction of a single base-pair deletion that generated a truncated PCPH oncoprotein (mutated PCPH). Recently, we isolated and characterized the cDNA for the human PCPH proto-oncogene and determined that in humans PCPH is a single-copy gene located in chromosome 14 (14q24.3). Pilot mRNA expression studies indicated that PCPH was expressed in the majority of normal organs tested, particularly in liver and kidney, but it appeared to be expressed either at low levels or not at all in tumor cells or cell lines derived from the high-expressing tissues. We have generated an antiserum against bacterial recombinant Syrian hamster PCPH. This antiserum recognizes both the normal and truncated, oncogenic Syrian hamster PCPH proteins and cross-reacts with the yeast, mouse, rat and human homologue proteins. Using this antibody, we have performed a study of PCPH expression in a larger sample of human neoplastic cell lines, including some derived from breast, nervous system, colon, lung and pancreas tumors. Results confirmed the frequent lack of PCPH expression in malignant cells and identified several immunoreactive forms of PCPH being differentially expressed in cells of diverse tissue origins.