Histidine is one of the basic (with reference to pH) amino acids due to its aromatic nitrogen-heterocyclic imidazole side chain. This amino acid is biochemically metabolized into the neurotransmitter histamine and the set of genes that produce the enzymes responsible for histidine biosynthesis are controlled by the well-studied histidine operon. The disruption of histidine biosynthesis in bacteria is the basis for the famous Ames test, used to determine the mutagenability of various chemicals. Histidine is incorporated into proteins and enzymes at a molar percentage of 2.1 compared to the other amino acids.