Mucosa-associated lymphoid tissue lymphoma is now classified as extranodal marginal zone B-cell lymphoma. We reviewed the current literature on the biological and genetic mechanisms that lead to the development and progression of this unusual lymphoma. Particular attention was given to the clinical and biological significance of the immunoglobulin genes rearrangement, that has been proposed and widely used both diagnostically and as a tool to monitor the response to antibiotic treatment.