Congenital metabolic diseases are a large group of genetic diseases. Most occur as a result of replacing a single nucleotide (SNP) of one gene. This gene encodes 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. The complaints also can occur as a result of the inability to synthesize one essential connections. Inborn errors of metabolism are now often referred to as congenital metabolic diseases or simply metabolic diseases (inborn error of metabolism).