The beaded lizard and the Gila monster, Heloderma horridum and Heloderma suspectum, were long treated as the only two species in a single genus. They share an unusual apparatus: venom glands in the lower jaw, ducts opening at the base of grooved teeth, and a chewing bite that works the venom into the wound. There is no pressurised injection. The venom contains kallikrein-like enzymes and bioactive peptides, and a bite produces intense pain, swelling and a drop in blood pressure.
The old claim that these were the only venomous lizards rested on the absence of comparable glands elsewhere. From 2005 onward, anatomical and transcriptomic work led by Bryan Fry described venom-gland tissue and venom-type proteins across a much broader group including monitors and several iguanian families, and proposed a single early origin of oral venom in a clade named Toxicofera. That framework has been contested, particularly over whether the secretions in many species function as venom at all.
The safe statement is narrower and still true: helodermatids are the lizards with an unambiguous, medically significant venom system. Their venom has been mined for pharmacologically useful peptides.