The B- and T-cell gene rearrangement test is one of the hallmarks of the molecular pathology diagnostic laboratories. Historically done with Southern blot and now adopted by many laboratories using PCR, the test is used as an aid in pathology diagnosis to distinguish between benign lymphoproliferative disorders, usually polyclonal, and clonally derived cell populations seen in malignant cell population. Clonally amplified gene rearrangements can be used for diagnosis and as tumor-specific markers to monitor recurrence.