Schematic model of MALT lymphomagenesis in the salivary gland. Clones detectable by PCR or Southern blot analysis develop in reactive MESA as a result of direct stimulation by a limited number of antigens and the occurrence of a premalignant oncogenic event. Initially, the clones are still dependent on antigen stimulation for growth and remain localized, although some may be capable of spreading to other salivary glands that are also involved by MESA. At some point, which may be many years after a clone first appears, additional oncogenic events occur that result in the clones becoming malignant lymphomas capable of widespread dissemination and growth in other organs.