Historically, naive pre-germinal center mantle zone B cells have been considered as the normal counterpart to MCL cells based on expression of CD5 and IgM/IgD, and early descriptions of most MCL with unmutated IGHV. However, recent studies have shown about 20-30% of MCL cases carry somatic mutations of IGHV gene, implying that the neoplastic cells in these cases either have been exposed to the germinal center environment or, alternatively, that somatic hypermutations have been acquired in a non-germinal center context. The gene discussed is CD5; the disease is mantle cell lymphoma.