Identification of causative mutations in the factor IX gene for families with hemophilia B can permit very accurate determination of carrier status of at-risk females in these families and provides options for prenatal diagnosis. To identify the mutation in a hemophilia B family, a blood sample is required from an affected individual (proband). Full mutation analysis of the factor IX gene is performed. After a mutation is detected in a proband, carrier testing and prenatal diagnosis may be offered to the family. In cases where a proband is not available for testing, analysis may be performed on a sample from an obligate carrier.