A conditional-ready gene trap cassette with FRT, F3, loxP, and Lox5171 sites upstream of a beta-geo reporter followed downstream by loxP, lox5171, FRT, and F3 sites in inverse orientation, was inserted into exon 1. This insertion is in the original gene-trapped configuration so that transcription is stopped at the beta-geo reporter, yielding a null allele, which can be excised into a functional allele through Flp- or Cre-mediated excision. (J:236978)