Down-regulated expression of the gene for iron-responsive element-binding protein 2 (IREB2) (Table 4), which regulates cellular iron homeostasis, might restrict intracellular growth of chlamydiae directly because iron restriction limits chlamydial infection (51) or indirectly through co-regulatory effects modulating tryptophan metabolism (52). Here, IREB2 is linked to chlamydia trachomatis infectious disease.