Instability in the ATCT variable number tandem repeat locus VWF.VNTR I in intron 40 of the von Willebrand factor gene

The von Willebrand factor gene intron 40 variable number tandem repeat VWF.VNTR I exhibits 10 alleles making it highly polymorphic and useful for parentage and forensic testing. 45 unrelated families (210 meiotic events) were tested for VWF.VNTR I alleles. One spontaneous mutation was observed in a family member. Haplotype analysis demonstrated that this mutation was due to a gain of one motif repeat by a paternal allele. Sequence analysis confirmed the difference in the number of motif repeats between the proband and the alleles expressed by the parents. This instability emphasizes the importance of demonstrating exclusion in at least two separate loci in parentage testing.