title: Haemophilia B in a girl.
abstract: Haemophilia B is extremely rare in females and so far 20 cases have been reported. A 9-year-old girl with severe haemophilia symptoms is described, who shows a very low level of factor IX activity (1.5%) and antigen (less than 10%), normal XX female karyotype and negative family history of bleeding tendency or consanguinity. This case is probably the fourth report of sporadic female haemophilia B without chromosomal aberration and the first one with quantitative analysis by immunoradiometric assay and enzyme-linked immunosorbent assay of factor IX antigen. This very low factor IX level and severe bleeding tendency of the patient would be consistent with the homozygous state, but since a double mutation is extremely rare the patient's laboratory findings can be more easily explained by extreme lyonization of the normal X-chromosome of a heterozygous carrier.
mesh_terms: D002648:Child; D004303:Dosage Compensation, Genetic; D004797:Enzyme-Linked Immunosorbent Assay; D005164:Factor IX / Q000032:analysis; D005260:Female; D002836:Hemophilia B / Q000235:genetics*; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D007621:Karyotyping
chemical_list: D005164:Factor IX