Proline codons 79 (CCA), 82 (CCT), 84 (CCA) and 87 (CCA) in exon 4 were changed to alanine (GCA, GCT, GCA, GCA) (p.P79A, p.P82A, p.P84A, p.P87A). The loxP site flanked neomycin resistance gene cassette that was inserted into intron 6 was removed through subsequent Cre-mediated recombination.