Thymosins are small proteins present in many animal tissues. They are named thymosins because they were originally isolated from the thymus, but most are now known to be present in many other tissues. Thymosins have diverse biological activities, and two in particular, thymosins α₁ and β₄, have potentially important uses in medicine, some of which have already progressed from the laboratory to the clinic. In relation to diseases, thymosins have been categorized as biological response modifiers.