Inborn errors of metabolism (IEM) are a group of disorders that causes abnormal function of biochemical pathways. Archibald Garrod des-cribed the first inborn error of metabolism in 1893. He described alkaptonuria in a patient whose urine turned black on standing and the development of arthritis in adult life.' Subse-quently, Garrod encapsulated the idea of IEM in 1908 with the concept of 'chemical indivi-duality'. Beadle and Tatum proposed the concept of one gene - one enzyme in 1945.2 Phenyl-ketonuria (PKU) was described in 1934 and amongst the first to be recognised as a cause of mental handicap with a biochemical basis.' Effective treatment for PKU with low pheny-lalanine diet was introduced in 1955. Molecular characterisation of genetic defects localised to alleles in various chromosomes were performed in the last two decades