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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®


Reviewed January 2008

What is the official name of the BTD gene?

The official name of this gene is “biotinidase.”

BTD is the gene's official symbol. The BTD gene is also known by other names, listed below.

What is the normal function of the BTD gene?

The BTD gene provides instructions for making an enzyme called biotinidase. This enzyme helps the body reuse and recycle biotin, a B vitamin found in foods such as liver, egg yolks, and milk. Biotinidase removes biotin that is bound to proteins in food, leaving the vitamin in its free (unbound) state. The body needs free biotin to activate enzymes called biotin-dependent carboxylases. These carboxylases are involved in many critical cellular functions, including the production and breakdown of proteins, fats, and carbohydrates.

In addition to processing biotin obtained from the diet, biotinidase recycles biotin within the body. As biotin-dependent carboxylases are broken down, they release a molecule called biocytin. Biocytin is a complex made of up biotin and a protein building block (amino acid) called lysine. Biotinidase splits this complex, making free biotin available for reuse by other carboxylase enzymes.

Researchers suspect that biotinidase may have several additional functions. This enzyme may transport free biotin through the bloodstream. It might also have the ability to attach biotin to certain proteins through a process called biotinylation. Within the nucleus, biotinylation of DNA-associated proteins called histones may help determine whether certain genes are turned on or off. It is unclear, however, whether biotinidase plays a role in regulating gene activity.

How are changes in the BTD gene related to health conditions?

biotinidase deficiency - caused by mutations in the BTD gene

More than 100 mutations in the BTD gene have been identified in people with biotinidase deficiency. Most of these mutations change single amino acids used to build biotinidase. These changes occur in critical regions of the enzyme, and reduce or eliminate the enzyme's activity.

Most BTD mutations cause profound biotinidase deficiency. This severe form of the disorder results when the activity of biotinidase is reduced to less than 10 percent of normal. Other mutations cause a milder form of the condition called partial biotinidase deficiency. These mutations reduce biotinidase activity to between 10 percent and 30 percent of normal. Without enough of this enzyme, biotin cannot be separated from proteins or recycled normally. As a result, not enough free biotin is available to activate biotin-dependent carboxylases and the body is less able to process important nutrients. These defects underlie the potentially serious medical problems associated with biotinidase deficiency.

Where is the BTD gene located?

Cytogenetic Location: 3p25

Molecular Location on chromosome 3: base pairs 15,601,351 to 15,647,639

The BTD gene is located on the short (p) arm of chromosome 3 at position 25.

The BTD gene is located on the short (p) arm of chromosome 3 at position 25.

More precisely, the BTD gene is located from base pair 15,601,351 to base pair 15,647,639 on chromosome 3.

See How do geneticists indicate the location of a gene? ( in the Handbook.

Where can I find additional information about BTD?

You and your healthcare professional may find the following resources about BTD helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the BTD gene or gene products?

  • Biocytin Hydrolase

See How are genetic conditions and genes named? ( in the Handbook.

What glossary definitions help with understanding BTD?

acids ; amino acid ; biocytin ; biotin ; breakdown ; carboxylase ; deficiency ; DNA ; egg ; enzyme ; gene ; hydrolase ; lysine ; molecule ; nucleus ; protein

You may find definitions for these and many other terms in the Genetics Home Reference Glossary (


  • Dobrowolski SF, Angeletti J, Banas RA, Naylor EW. Real time PCR assays to detect common mutations in the biotinidase gene and application of mutational analysis to newborn screening for biotinidase deficiency. Mol Genet Metab. 2003 Feb;78(2):100-7. (
  • Hymes J, Stanley CM, Wolf B. Mutations in BTD causing biotinidase deficiency. Hum Mutat. 2001 Nov;18(5):375-81. Review. (
  • McMahon RJ. Biotin in metabolism and molecular biology. Annu Rev Nutr. 2002;22:221-39. Epub 2002 Jan 4. Review. (
  • NCBI Gene (
  • Wolf B, Jensen KP, Barshop B, Blitzer M, Carlson M, Goudie DR, Gokcay GH, Demirkol M, Baykal T, Demir F, Quary S, Shih LY, Pedro HF, Chen TH, Slonim AE. Biotinidase deficiency: novel mutations and their biochemical and clinical correlates. Hum Mutat. 2005 Apr;25(4):413. (
  • Wolf B. Biotinidase: its role in biotinidase deficiency and biotin metabolism. J Nutr Biochem. 2005 Jul;16(7):441-5. Review. (
  • Wolf B. Biotinidase Deficiency: New Directions and Practical Concerns. Curr Treat Options Neurol. 2003 Jul;5(4):321-328. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: January 2008
Published: October 20, 2014