• factor X deficiency;
  • acceptor splicing site;
  • polypyrimidine tract;
  • spliceosomes

We investigated the molecular defect underlying congenital factor X (FX) deficiency in a Japanese patient. A novel three-base-pair deletion was identified within intron D of the FX gene. An allele-specific restriction analysis showed the propositus was homozygous and her two daughters were heterozygous for the deletion. It was not detected in 53 unrelated Japanese (106 alleles), indicating a probable cause of the FX deficiency. The deletion resides within a polypyrimidine tract of the acceptor splicing site where U2 snRNP binds to form spliceosomes. The defect could alter the formation of spliceosomes, resulting in incorrect splicing and decreased FX production.