Mild haemophilia B from a novel missense mutation Ser350Ala of the F9 gene.

Haemophilia B is an X-linked recessive bleeding disorder caused by mutations in the F9 gene on Xq27.1, which lead to deficient coagulation factor IX (FIX). The mild form of haemophilia B has been known to be underdiagnosed due to mild clinical symptoms and minimally prolonged activated partial thromboplastin time. We herein describe a sporadic case of mild haemophilia B from a novel missense mutation of F9. The proband was a 4-year-old boy with a mild bleeding history. He had no family history of bleeding tendency. Coagulation screening tests revealed prolonged aPTT at 42.6s (STA-PTT Automate, reference range, 29.1-41.9s) and a decreased FIX activity at 13% in factor assays. Molecular genetic analysis of F9 revealed that he was hemizygous for a missense mutation, c.1048T>G (p.Ser350Ala), which has not been reported previously. His mother was a carrier of the mutation. This case represents a novel missense mutation from non-CpG transversion of F9.