Thallium is known as the poisoner’s poison; it is quite toxic. In ionic form, thallium exists as Tl+ or Tl3+. Tl+ has an ionic radius that is similar to that of K+, an important biological ion; both ions have the same charge. As a result, Tl+ can mimic K+ in the body, substituting for it in various pathways. Tl+ binds strongly to sulfur, which is found in many proteins. When Tl+ binds to proteins, it denatures (destroys) them, which leads to many problems.