A loxP site was inserted upstream of exon 3. Exon 4 was replaced with a modified exon 4 that lacked 93 bps encoding amino acids 58 through 88 of the proline rich domain (mdeltapro). An FRT-flanked neo cassette with a 3' loxP site was inserted downstream of the modified exon 4. Flp-mediated recombination removed the neo cassette.