Inborn error of metabolism (aka congenital metabolic disease, inherited metabolic disease) are a large class of genetic diseases, involving congenital disorders of metabolism. The majority are due to defects of single genes, that code for enzymes that facilitate conversion of various substances (substrates) into others (products). In most of the disorders, problems arise due to accumulation of substances which are toxic or interfere with normal function, or to the effects of reduced ability to synthesize essential compounds.

• Disorders of carbohydrate metabolism, e.g. glycogen storage disease
• Disorders of amino acid metabolism, e.g. phenylketonuria, maple syrup urine disease (aka branched chain ketonuria), glutaric acidemia type 1
• Urea cycle disorder or urea cycle defects, e.g. carbamoyl phosphate synthetase 1 deficiency
• Disorders of organic acid metabolism (organic acidurias), e.g. alcaptonuria, 2-hydroxyglutaric acidurias
• Disorders of fatty acid oxidation and mitochondrial metabolism, e.g. medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD)
• Disorders of porphyrin metabolism, e.g. acute intermittent porphyria
• Disorders of purine or pyrimidine metabolism, e.g. Lesch-Nyhan syndrome
• Disorders of steroid metabolism, e.g. lipoid congenital adrenal hyperplasia, congenital adrenal hyperplasia
• Disorders of mitochondrial function, e.g. Kearns-Sayre syndrome
• Disorders of peroxisomal function, e.g. Zellweger syndrome
• Lysosomal storage disorders, e.g. Gaucher's disease, Niemann Pick disease
• Disorders of cholesterol synthesis, e.g. Smith-Lemli-Opit syndrome (aka SLOS, or 7-dehydrocholesterol reductase deficiency)