A neoplasm of follicle center B cells which has at least a partial follicular pattern. Follicular lymphomas comprise about 35% of adult non-Hodgkin lymphomas in the U.S. and 22% worldwide. Most patients have widespread disease at diagnosis. Morphologically, follicular lymphomas are classified as Grade 1, Grade 2, and Grade 3, depending on the percentage of the large lymphocytes present. The vast majority of cases (70-95%) express the BCL-2 rearrangement [t(14;18)]. Histological grade correlates with prognosis. Grades 1 and 2 follicular lymphomas are indolent and grade 3 is more aggressive (adapted from WHO, 2001).