Transgenic mice were generated in which 5 kb of the 5′ flanking promoter region of the human Factor IX (FIX) gene fused to various FIX constructs (gene, minigene and cDNA) were stably integrated in the germ line. Several transgenic mouse lines expressed high circulating levels of active and correctly processed recombinant human FIX. The presence of at least one FIX intron had a positive effect on the expression. The FIX transgenes were expressed in a tissue‐specific manner in the liver of transgenic mice. By crossing transgenic mice synthesizing FIX with others prone to develop hepatoma, progeny which co‐express the transgenes in hepatocytes were obtained. Hepatoma‐derived cell lines were shown to have a differentiated phenotype and secrete active human FIX for many generations.

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