A mutant embryonal carcinoma (EC) cell line, NR1-6, has been isolated subsequent to infection with a retroviral insert containing a neomycin resistance cassette (Neo(r)) (1). This variant cell line has only a single insertion site, but is unique in its morphology, tumorigenicity, and differentiative potential (2). In order to understand the exact nature of the mutational event, we have sequenced the regions immediately flanking the insertion site (encompassing over 18 kb). Searches for homology in GenBank using the GCG and Blast programs have revealed homology to only a few small conserved regions (4 B1 repetitive elements approximately 150 bp long and a CA/GT dinucleotide repeat). Interestingly, B1 repetitive elements have been reported to play a role in germ cell differentiation. One of these elements is immediately 5' upstream of the insertion site and it appears as if the retroviral insertion may be responsible for activating or repressing the expression of this element. The CA/GT repeat has not yet been mapped within the mouse genome. Thus, we have identified a novel mouse locus which apparently regulates a number of cellular phenotypes.

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