Thiazolidinediones, which are being developed for the treatment of insulin resistance and type 2 diabetes mellitus, bind and activate peroxisome proliferator-activated receptor gamma, a nuclear receptor that regulates the expression of several genes involved in metabolism. This receptor controls adipocyte differentation, lipid storage, and insulin sensitisation. Besides metabolic activities, thiazolidinediones have effects as diverse as the control of host defence, cell proliferation, and tumorigenesis.