We are using somatic cell and molecular genetic techniques to study loci that regulate developmental and tissue-specific patterns of gene expression. Somatic hybrids formed by fusing cells of different types generally fail to express the tissue specific products of either parent, a phenomenon termed extinction. Two discrete genetic loci, Tse-1 and Tse-2, have been shown to contribute to the extinction of distinct sets of liver- specific genes in hepatoma-fibroblast hybrids. This is a proposal to investigate one of these loci, Tse-2, in detail. The aim of this project is to determine the means by which Tse-2 mediates extinction of a group of liver genes, including albumin and alcohol dehydrogenase. Tse-2 has been mapped to murine chromosome 1, although stable monochromosomal hybrids are not available to facilitate study of Tse-2. The initial phase of this project will use microcell-mediated chromosome transfer to generate stable monochromosomal hybrids containing an active Tse-2 locus. After karyotypic and Southern blot analysis of extinguished clones, they will be used to study the Tse-2+ phenotype in detail, and to investigate the relationship between Tse-2 and other known regulators of transcription of the affected genes. These hybrids will also be used as starting materials for cDNA subtractive cloning of the Tse-2 transcript.