Mutations in the "BTD" gene cause biotinidase deficiency. Biotinidase is the enzyme that is made by the "BTD" gene. Many mutations that cause the enzyme to be nonfunctional or to be produced at extremely low levels have been identified. Biotin is a vitamin that is chemically bound to proteins. (Most vitamins are only loosely associated with proteins.) Without biotinidase activity, the vitamin biotin cannot be separated from foods and therefore cannot be used by the body. Another function of the biotinidase enzyme is to recycle biotin from enzymes that are important in metabolism (processing of substances in cells). When biotin is lacking, specific enzymes called carboxylases cannot process certain proteins, fats, or carbohydrates. Specifically, two essential branched-chain amino acids (leucine and isoleucine) are metabolized differently.