Other non-inhibitory ligands, which bind the active site and stabilize some mutants in the cells, have been described, 11-cis-retinal and tetrahydrobiopterin, which are the natural cofactors of rhodopsin [73] and phenylalanine-hydroxylase respectively, for retinitis pigmentosa and phenylketonuria [74,75,76]. The gene discussed is RHO; the disease is retinitis pigmentosa.