Multiple XbaI polymorphisms for carrier detection and prenatal diagnosis of haemophilia A.

Three XbaI restriction fragment length polymorphisms (RFLPs) can be detected using the factor VIII-intron 22 probe (p482.6) in a XbaI-KpnI double digest of genomic DNA. The XbaI (A) site had been reported by Wion et al (1986) to be in intron 22, while the two additional sites. XbaI (B) and XbaI (C), are shown here to be X-linked and close to the XbaI (A) site. The frequencies of heterozygosity for these three sites are 0.49, 0.18 and 0.30 respectively. In 75 females the observed heterozygosity rate for the XbaI (A) site is 0.41 and this increased to 0.57 with the two additional sites. Care should be exercised when interpreting the XbaI RFLPs, since the 1.4 kb XbaI/KpnI fragment and the 4.8 kb XbaI fragment are associated with both positive XbaI (A) and XbaI (B) sites. By the combined use of the multiple XbaI polymorphisms with the BclI site in intron 18, the carrier detection rate would increase to 67%. Four prenatal diagnoses had been performed using the multiple XbaI polymorphisms.