Phenylalanine and tyrosine represent precursors in the pathway of the synthesis of catecholamines, thus alterations in their metabolism and a diminished conversion of Phe to Tyr by the enzyme phenylalanine-hydroxylase (PAH), could lead to a reduced cognitive performance [8,14]; by the way the metabolism of these two amino acids in the setting of HIV infection has not been extensively investigated as that of tryptophan. This evidence concerns the gene PAH and HIV infectious disease.