Based upon current research results, the world's most venomous lizard is the Gila monster Heloderma suspectum, native to the southwestern USA and northwestern Mexico. LD50 (lethal dose) values for its venom as low as 0.4 mg/kg have been recorded when administered intravenously in mice. This is comparable to the toxicity of the venom of the western diamondback rattlesnake Crotalus atrox. LD50 values as low as 1.4 mg/kg have been recorded from the Gila monster's only close living relative, the Mexican beaded lizard H. horridum. Happily, human deaths from these species are few, because only relatively small amounts of venom are injected when the lizard bites. For many years, these two species were believed to be the world's only venomous lizards alive today, but in recent times scientists have been surprised to discover that several species of monitor lizard and iguana are also venomous. However, the precise levels of toxicity and the effects of the various venom constituents have yet to be fully determined for these latter species.