Specific activity of wild-type hF.IX synthesized in murine liver, as a function of levels of protein expression. Mice with circulating levels less than 10 μg/mL (n=16) have a specific activity of 341 U/mg ± 102 U/mg protein, whereas mice with circulating levels greater than 10 μg/mL (n = 4) have specific activity of 208 U/mg ± 52 U/mg protein. These values are different at a statistically significant level (P=.022).