Transient Cre expression in embryonic stem cells whose endogenous gene bore previous modifications, including loxP sites flanking a 2000-base pair segment, resulted in deletion of exons 5 through 11, which encode most of the region common to all three receptor isoforms; this mutant allele also contains a previously introduced frt-flanked Pgk-neomycin phosphotransferase cassette in intron 11 and a deletion of 3.9 kb from intron 4. (J:93288)