An indolent, extranodal type of non-Hodgkin lymphoma composed of small B-lymphocytes (centrocyte-like cells). The gastrointestinal tract is the most common site of involvement. Other common sites of involvement include lung, head and neck, ocular adnexae, skin, thyroid, and breast. Gastric involvement is associated with the presence of H. pylori infection. (WHO, 2001)