A sequence between 1 and 9 kb upstream of the P1 start site was deleted and replaced with a floxed PGK-neomycin resistance gene. Southern blot and PCR confirmed recombination. RT-PCR of mutants indicated antisense expression was not affected and ectopic sense transcription was detected upstream of Xist, as well as elevated transcription within the Xist locus. (J:85305)