The FTCD gene provides instructions for making an enzyme called formiminotransferase cyclodeaminase. This enzyme is found mainly in the liver, with smaller amounts in the kidneys. In males it is also found in the testes.

Formiminotransferase cyclodeaminase is called a bifunctional enzyme because it performs two functions. Specifically, it is involved in the last two steps in the breakdown (metabolism) of the amino acid histidine, a building block of most proteins. It also plays a role in producing one of several forms of the vitamin folate, which has many important functions in the body.

One of the enzyme's functions, called formiminotransferase, breaks down a molecule called N-formiminoglutamate in the process of histidine metabolism. Part of the N-formiminoglutamate molecule, called the formimino group, is incorporated into another molecule called formiminotetrahydrofolate. The amino acid glutamate is also produced in this reaction.

The second function of the enzyme, called cyclodeaminase, breaks down formiminotetrahydrofolate to a molecule called 5,10-methenyltetrahydrofolate and ammonia. 5,10-methenyltetrahydrofolate is one of several tetrahydrofolate molecules involved in the production (synthesis) of many important molecules in the body, such as purines and pyrimidines (the building blocks of DNA and its chemical cousin, RNA) and amino acids. The tetrahydrofolates carry small molecules called one-carbon units that are needed for the synthesis of these larger molecules.