Exon 4 was replaced with a loxP site and a modified exon 4 that contains nucleotide substitutions resulting in the amino acid substitution of cysteine for arginine at position 262 (R262C), mimicking a mutation found in some congenital fibrosis of the extraocular muscles type 3 (CFEOM3) patients. An frt and loxP flanked neo cassette downstream of the modified exon 4 was removed by flp mediated recombination leaving the modified exon 4 floxed. Reduced protein expression was confirmed by western blot analysis on extracts from homozygous and heterozygous mice.