| Disorder | Defective / deficient protein | Clinical features | Treatment |
| Amino acid metabolism - Maple syrup urine disease | Branched chain keto-acid dehydrogenase | Sugary smelling urine, breathing, feeding problems, mental retardation, death. | Dietary restriction of branched chain amino acids |
| Carbohydrate metabolism- Galactosemia | Galactose-1-phosphate uridyl transferase | Inability to digest milk and milk products, vomiting, enlarged liver, jaundice, cataract, mental retardation and death if untreated. | Exclusion of galactose from the diet |
| Nucleic acid metabolism- Adenosine deaminase deficiency | Adenosine deaminase | Toxicity to lymphocytes, causing severe combined immunodeficiency disease. Fatal if left untreated. | Gene therapy |
