Lizard heads called the parietal eye, a basal ("primitive") feature also present in the tuatara. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps them detect predators stalking it from above. Until 2006 it was thought that among lizards, only the Gila monster and the Mexican beaded lizard were venomous. However, several species of monitor lizards, including the Komodo dragon, produce powerful venom in their oral glands. Lace monitor venom, for instance, causes swift loss of consciousness and extensive bleeding through its
Mexican beaded lizard (swelling), weakness, sweating, and a rapid fall in blood pressure. Beaded lizards are immune to the effects of their own venom. Some compounds in its venom have been shown to have pharmacological properties relating to the possible treatment of diabetes, Alzheimer's disease, and even HIV. One compound, a hormone named exendin-3, is marketed by Amylin Pharmaceuticals as the drug exenatide. A study in 1996 revealed that it binds to cell receptors in breast cancer cells and may stop the growth of lung cancer cells. The beaded lizard becomes sexually mature at six to eight years and mates between September and