Schematic presentation of the different mRNA transcript species in patients II-1 and II-3. Maternal allele produces reduced amount of a 3.9-kb mRNA that contains the Arg661 → Stop mutation in exon XIV. The paternal allele produces several mRNA variants. One of the transcripts contains sequence from intron C, which creates a premature stop codon in the mRNA. Two of the transcripts show exon skipping. In a more abundant transcript, exon 3 has been skipped and in a more rare transcript, both exons 3 and 4 have been skipped. Most importantly, the paternal allele also produces a minute amount of correctly spliced mRNA, thus facilitating the production of some active enzyme and resulting in a mild disease phenotype.