Mutation in the splice donor site of intron 18. The mutant produces two transcripts. In the first, intron 18 is not removed, such that it encodes a protein in which the normal sns sequence ends at amino acid residue I1163 and is followed by 46 additional amino acids that result from translation of intronic sequence. In the second, a cryptic splice donor site that is three nucleotides after the correct splice site is used, resulting in a protein in which the sns sequence also diverges at amino acid residue I1163.