Exon 4 was replaced with a modified version in which coding nucleotide 205 was changed from C to T (c.205C>T). This results in the amino acid substitution of arginine with cysteine at position 69 (p.R69C) in the encoded protein. In vivo, however, the mutation results in aberrant splicing, with the majority of transcripts skipping exon 4 (which results in a frameshift and premature termination of translation). The mutation corresponds to one observed in some human X-linked myotubular myopathy (MTM) patients. A loxP site flanked neomycin resistance gene cassette was inserted into intron 4.