An FRT flanked neo with a 5' loxP site was inserted upstream of exon 11. A modified exon 11 (exon 11a) was inserted downstream of the endogenous exon. It contains a stop codon at the 5' end and a loxP site at the 3' end. Flp mediated recombination removed the neo cassette leaving exons 11 and 11a floxed. Severely reduced protein expression was confirmed by western blot analysis on skin and heart extracts. This is a hypomorphic allele.