The genes for the enzymes which catalyze synthesis of the amino acid tryptophan are clustered together on a chromosome. Along with their promoter and operator they form the trp operon. As the genes are transcribed and then translated, tryptophan levels increase. Tryptophan, acting as a corepressor, binds to a repressor molecule. This complex binds to the chromosome, blocking RNA polymerase and thus preventing further tryptophan production. These are called repressible enzymes because the presence of their end product, tryptophan, inhibits, or represses, their production.