A loxP-FRT-neo-FRT cassette was introduced to intron 3, a 2X hemagglutinin (HA) tag was placed in front of the first immunoglobulin (Ig) domain in exon 4, and a loxP site was placed in intron 4. Flp-mediated recombination removed the FRT-flanked neo cassette. (J:287233)