Mutation of the repeat number of the HPRTB locus and structure of rare intermediate alleles

During routine paternity testing a mutation of a paternal allele at the HPR TB locus was observed. The opportunity was taken to analyse this mutation a t a molecular level. The repent sequence is flanked bq an imperfect repeat sequence and this region could he involved in the mutation mechanism. For t his reason, we also examined the structure of "intermediate" alleles. Seque ncing confirmed the insertion of a perfect repeat motif and revealed a dele tion of a dinucleotide some 50 nucleotides downstream from the repeat seque nce for the intermediate alleles. It is likely that these intermediate alle les are rare biallelic deletion polymorphisms and are probably not involved in the mutation or variation mechanism of this locus.