tokens: Three exon 2 mutations were identified , including a single nucleotide deletion ( 2692delA ) of codon 349 introducing a frameshift and premature termination codon , a 2 bp deletion ( 2673 - 2674delCT ) that results in introduction of a stop codon at amino acid 343 , and a G - - > A substitution in codon 429 ( 2931G - - > A ) introducing a premature termination codon .