Galactosemia classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Dayana Davidis, M.D. [2]


Galactosemia consists of four types of disorders resulting from abnormal activity of the enzymes involved in galactose metabolism. Type I/ Classical is the most serious form of the disease.


Galactosemia refers to a group of autosomal recessive disorders of galactose metabolism. They are classified into the following types based on the enzymatic activity [1]

Type I/Classical

  • Defects in both copies of the gene that codes for the enzyme galactose-1-phosphate uridyl transferase or GALT.[2].
  • Most common and severe form.
    • Duarte galactosemia
      • Variant of classic galactosemia.
      • Galactose metabolised to some extent as GALT not completely absent
      • Less severe symptoms than other forms

Type II

  • Defect in both copies of the gene that codes for the enzyme galactokinase or GALK.[3]
  • Less common and harmful than classic galactosemia.
  • Does not generally lead to hepatomegaly or brain damage.

Type III

  • Alterations in the gene coding for epimerase enzyme.[4] This is the mildest type.

Type IV

  • This type has only been discovered recently.
  • Abnormal changes in the galactose mutatrose enzyme, encoded by the GALM gene, have been proven to cause the disorder. [5]


  1. Banford S, McCorvie TJ, Pey AL, Timson DJ (2021). "Galactosemia: Towards Pharmacological Chaperones". J Pers Med. 11 (2). doi:10.3390/jpm11020106. PMC 7914515 Check |pmc= value (help). PMID 33562227 Check |pmid= value (help).
  2. Coelho AI, Rubio-Gozalbo ME, Vicente JB, Rivera I (2017). "Sweet and sour: an update on classic galactosemia". J Inherit Metab Dis. 40 (3): 325–342. doi:10.1007/s10545-017-0029-3. PMC 5391384. PMID 28281081.
  3. Holden HM, Thoden JB, Timson DJ, Reece RJ (2004). "Galactokinase: structure, function and role in type II galactosemia". Cell Mol Life Sci. 61 (19–20): 2471–84. doi:10.1007/s00018-004-4160-6. PMID 15526155.
  4. Timson DJ (2006). "The structural and molecular biology of type III galactosemia". IUBMB Life. 58 (2): 83–9. doi:10.1080/15216540600644846. PMID 16611573.
  5. Banford S, Timson DJ (2021). "The structural and molecular biology of type IV galactosemia". Biochimie. 183: 13–17. doi:10.1016/j.biochi.2020.11.001. PMID 33181226 Check |pmid= value (help).

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