Consistent with evidence that supplementation of BH4 [116,123,124,125,126] and an increased BH4/oxidized BH4 ratio [31,121,123] enhance eNOS activity, oral BH4 administration promotes eNOS activity, lowers lung O2.− levels, and reverses established CH-induced PH [34]. This evidence concerns the gene NOS3 and cyclic hematopoiesis.