468 bp of endogenous sequence in exon 1 was replaced with a 39 bp intracisternal A particle (IAP) sequence resulting in a new translation initiation methionine codon and substitution of 7 new amino acids for 10 endogenous amino acids at the N-terminus (MRQLCRGRVL becomes MWGRLPT). No gain of function effect is observed as in other spontaneous mutations derived from IAP insertion.