Seeming homozygosity in type-IIB von Willebrand's disease due to a polymorphism within the sequence of a commonly used primer.

In an isolated type-IIB von Willebrand's disease patient we detected a Val553-->Met substitution. The patient seemed to be homozygous, although this substitution was absent in both asymptomatic parents. Since two identical de novo mutations are highly unlikely, we tested for unsuccessful amplification of one allele. We found a novel polymorphism in the complementary sequence of a primer widely used for selective amplification of the von Willebrand factor gene.