To assess the levels of Tetrahydrobiopterin in the retina, three to five pools of retinas are collected from WT and hph-1mice at postnatal age 7, 14, and 22 and evaluated by LC-MS/MS. LC-MS/MS analysis confirm a significant decrease by approximately 90% in the concentration levels of Tetrahydrobiopterin in retinal tissue from hph-1 mice (0.0009±0.0006; p<0.0001, 0.01±0.001; p<0.0001 and 2.45±0.40; p<0.005) compare to the WT group (0.014±0.001, 0.092±0.01, and 23.13±6.44) at P7, P14, and P22, respectively.