BH₄ suppresses atherosclerotic progression in apoE-KO/eNOS-Tg mice. Representative photographs of Sudan III–stained, longitudinally opened aortas from BH₄-treated (b) and untreated (a) apoE-KO/eNOS-Tg mice on a high-cholesterol diet for 12 weeks. Atherosclerotic lesion formation was remarkably suppressed by BH₄ administration in apoE-KO/eNOS-Tg mice. (c) Quantitative analysis of atherosclerotic lesion size in aortas from apoE-KO/eNOS-Tg mice. In BH₄-treated apoE-KO/eNOS-Tg mice, atherosclerotic lesion size was significantly smaller than in untreated mice. *P < 0.05 vs. untreated apoE-KO/eNOS-Tg males; **P < 0.01 vs. untreated apoE-KO/eNOS-Tg females. (d) Quantitative analysis of atherosclerotic lesion size in aortas from apoE-KO mice. BH₄ treatment also decreased atherosclerotic lesion size in apoE-KO females; however, the reduction was comparatively smaller than in apoE-KO/eNOS-Tg mice. ApoE-KO males did not show a significant reduction of lesion size. *P < 0.05 vs. untreated apoE-KO females.