Thyroglobulin type-1 domain is a structural element found in a variety of functionally unrelated proteins. It may occur singly, as in insulin-like growth factor binding proteins, or multiply up to eleven, as in thyroglobulin. Some of these type-1 domain containing proteins have been shown to have the ability to inhibit either cysteine or cation-dependent proteinases. For these inhibitory proteins we proposed the term thyropins. These inhibitors originate from a variety of organisms and presumably have different biological functions. One of those whose function has been studied is the p41 invariant chain fragment. In vivo it associates with the major histocompatibility complex (MHC) class II molecule. The evidence supporting its involvement as a controlling factor in the process of antigen presentation is described.