Coagulation factor IX deficiency causes hemophilia B in humans. We have used gene targeting to develop a coagulation factor IX-deficient (factor IX-knockout) mouse strain. Mouse embryonic stem (ES) cells were targeted by a socket-containing vector that replaces the promoter through exon 3 of the factor IX gene by neoΔHPRT, which is a functional neo gene plus a partially deleted hypoxanthine phosphoribosyl transferase minigene. Chimeric mice generated using these socket-containing ES cells transmitted the targeted factor IX gene to their female offspring. Male offspring from these females were characterized and shown to exhibit a phenotype similar to hemophilia B. This factor IX-deficient mouse strain will be useful for studying gene therapy methods and structure-function relationships of recombinant factor IX proteins in vivo.