The pem gene (cDNA) was isolated by the subtraction hybridization technique from a murine T-lymphoma cell clone, SL12.4. The name pem is derived from the finding that it is expressed in placenta and during embryogenesis. Pem mRNA is also present in immortalized cell lines from many cell lineages, including lymphomas, myeloid tumors, erythroleukemias, immortalized fibroblast, epithelial, and neuronal cell lines, and embryonal carcinoma cells. In contrast, pem is not detectably expressed in any adult tissue tested, with the exception of uterine tissue from pregnant mice. The deduced amino acid sequence of pem reveals that the putative protein product is unique, although a limited homology with thymosin is apparent. Thus, pem is a novel gene which appears to be normally restricted in its expression to embryonic development, but is also activated in immortalized cells. Studies are proposed to analyze in detail the locality of pem transcripts in embryonic and extra-embryonic tissue by in situ hybridization, blot analysis, and uterine function will be tested for their effects on pem expression in vitro and in vivo. Preliminary results indicate that pem mRNA levels are regulated in cell lines which differentiate in vitro. It will be determined whether regulation is exerted at the level of determination and/or differentiation, and whether pem expression (or lack of) is a prerequisite for differentiation. We will analyze whether pem mRNA accumulation is regulated at the transcriptional or post-transcriptional level. Genomic pem clone(s) will be isolated and sequenced to provide the tools for future studies examining the detailed molecular mechanisms which regulate pem gene expression. Lastly, the possible role of pem in the processes of immortalization and transformation will be investigated by transfection experiments. The pem gene is a unique gene with a provocative pattern of expression. It is important to learn more about the role it plays in development, its usefulness as a marker of immature, immortalized or malignant cells, and its potential role in the process of malignancy.