Congenital metabolic diseases are a large group of genetic diseases. Most occur as a result of the replacement of a single nucleotide leading to a single nucleotide polymorphism or ‘snp’ in a gene. This gene encodes for an enzyme, which facilitates (stimulates) the conversion of a particular substance (substrate) in another product (intermediate). Most disorders occur as a result of accumulation of substances that are toxic or interfere with the normal function. Disorders can also occur as the result of an inability to synthesize essential substances. Inborn errors of metabolism are now often referred to as congenital metabolic diseases or simply metabolic diseases.