The structure of factor IX gene was analyzed in a hemophilia B patient with inhibitor. Genomic DNA, digested with a variety of restriction endonucleases, was hybridized with the cDNA and various genomic factor IX probes. A large subtotal deletion of the gene was observed. The borders of the deletion span from a .apprx. 125 nucleotide region within the last exon to an unknown domain at least 7.5 kb upstram from the first exon: it thus involves .apprx. 33 kb of the factor IX locus. The abnormal gene was inherited by the daughter of the propositus, who showed both the normal and the deleted allele.