Clearance of infused human factor IX. Mice (n = 3) producing wild-type factor IX (▾, gray broken line, error bars up only), R333Q-hFIX (, solid line, error bars down only), or no factor IX (knockout; ▪, error bars down) were injected at zero time with 4 μg recombinant human factor IX via the jugular vein. Samples were collected from the tail vein at serial time points for 15 minutes after infusion, and the survival of the infused Thr148 isoform factor IX was determined by A-1 antibody ELISA. The concentration at the first time point (30 seconds) is defined as 100%.