The targeting vector contains loxP sites flanking exon 1, including the initiation codon. An FRT-flanked neomycin resistance (neo) cassette fused to an internal ribosomal entry site (IRES) and a splice donor (SD) sequence, followed by a third loxP site, are placed downstream of exon 1. Flp-mediated recombination removed the FRT-flanked neo-IRES-SD cassette. (J:213820)