Increased hemoglobin and hematocrit values in Hif-p4h-2^gt/gt/P4h-tm^−/− double gene-modified mice. The hemoglobin (A), hematocrit (B), serum Epo (C), and kidney Epo mRNA (D) values are shown for controls (these include wild-type, Hif-p4h-2^+/+/P4h-tm^+/−, Hif-p4h-2^+/gt/P4h-tm^+/+, and Hif-p4h-2^+/gt/P4h-tm^+/− mice), Hif-p4h-2^+/gt/P4h-tm^−/− mice, and Hif-p4h-2^gt/gt/P4h-tm^−/− mice. Statistical significance is shown for comparisons with control mice (*P < .05, ***P < .00005). n = 21 for controls, n = 6 for Hif-p4h-2^+/gt/P4h-tm^−/− mice, and n = 6 for Hif-p4h-2^gt/gt/P4h-tm^−/− mice. Error bars represent SEM.