The study the molecular nature of the mouse genetic locus, Fis-1, has been continued. This locus has been shown to be the site of integration of Friend MuLV in some lymphomas appearing after inoculation of several inbred mouse strains with the ecotropic Friend MuLV. In about 5% of approximately 100 tumors examined, copies of Friend MuLV were found integrated within a 1.5kb region of the mouse genome (Fis-1). From a mouse genomic library constructed in lambda virus, 35kb of mouse genomic sequence surrounding the Fis-1 locus were isolated. These mouse sequences were present in two recombinant lambda phages. The mouse sequence inserts in the lambda phage were subcloned into plasmids and fragments from 7 to .3kb were obtained. Each subcloned fragment was analyzed for unique or repetitive mouse sequence content by Southern blot analysis. Subclones containing a total of about 15kb of unique mouse sequence were further utilized to test for expression in RNA isolated from mouse tumors. Gene mapping experiments show that Fis-1 is extremely closely linked to Int-2, a preferred integration region for mouse mammary tumor virus in mammary carcinomas. Studies at the RNA and DNA level prove that these loci are distinct.
