Murine embryonal carcinoma (EC) cells do not normally express Moloney murine leukemia virus genes. Earlier, rare EC cell lines were isolated that expressed proviral neomycin resistance (neo) gene. This expression was dependent on cellular enhancer or promoter sequences that flank the proviral integration site. Four such integration sites, designated as Mint (for Moloney murine leukemia virus integration and expression sites in EC cells), have been mapped on mouse chromosomes. Minta, Mintb, Mintc and Mintd are unlinked and mapped on different chromosomes (Chr), Chr 10, Chr 1, Chr 5 and the X Chr, respectively. None of these loci appear to be linded to any known Mo-MuLV proviral integration sites previously mapped. These enhancer and promoter loci may represent a new set of genes active in undifferentiated embryonic cells.
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