As IFN-γ decreases the availability of L-tryptophan and can lead to resolution of chlamydial infections [58, 59], the ability to block its production may be an important virulence determinant that allows the organism to form persistent subclinical infections, particularly when these pathogens appear to lack the ability to synthesise tryptophan with the absence of the tryptophan biosynthesis operon. The gene discussed is IFNG; the disease is infection.