In the article by Waddington et al entitled “In utero gene transfer of human factor IX to fetal mice can induce postnatal tolerance of the exogenous clotting factor,” which appeared in the February 15, 2003, issue of Blood (Volume 101:1359-1366) and in the article by Waddington et al entitled “Permanent phenotypic correction of hemophilia B in immunocompetent mice by prenatal gene therapy,” which appeared in the November 1, 2004, issue of Blood (Volume 104:2714-2721), human factor IX concentrations in murine plasma samples were measured using a commercial sandwich ELISA (Roche Diagnostics, Mannheim, Germany). A standard curve was constructed based on doubling serial dilutions of standard human factor IX:Ag standard supplied in the kit. A second-order polynomial regression curve was fitted to the plot of absorbance versus factor IX concentration, and sample concentrations were calculated from the regression equation. But due to a typographical error in the manufacturer's instructions, the concentration of the standard was interpreted to be half its actual value. Therefore all sample values were, in fact, reported erroneously as half their true concentration. Nevertheless, all conclusions drawn in the studies remain valid as this error was consistent throughout.