Full scanning of the factor IX gene by means of denaturing gradient gel electrophoresis enabled us to determine the molecular defects in 48 out of 49 hemophiliacs and to evaluate the spectrum of factor IX mutations in the French population. Our results further document the high molecular heterogeneity of the disease and the efficiency of this rapid screening method for disease-causing mutations. This direct approach, which is based on computer-aided analysis of the whole coding, promoter and exon-flanking factor IX gene sequences, proved to be helpful for carrier detection and prenatal diagnosis in most hemophilia B families, including sporadic cases. Moreover, we were able to identify 24 novel molecular defects of various natures in the factor IX gene.