Factors IX and X are plasma glycoproteins important in the middle phase of the coagulation cascade, and a bleeding disorder of variable severity results from abnormalities in the expression of either gene encoding these proteins. Nearly 380 unique molecular mechanisms cause factor IX deficiency, or haemophilia B, but only a limited number of mutations causing congenital factor X deficiency have been characterized to date. In this study enzymatic amplification has been used to examine the molecular basis for factor IX deficiency in two patients and factor X deficiency in two patients. Genomic DNA was isolated from each patient and synthetic oligonucleotide primers were used in the polymerase chain reaction to amplify each exon, splice junction and polyadenylation site. Amplified DNA was then cloned into pUC18 and sequenced. Five novel point mutations were identified, two occurring in the eighth exon of the factor IX gene and three in the eighth exon of the factor X gene. One of the haemophilia B mutations and one of the factor X mutations altered homologous histidine residues near the serine of the catalytic triad.