Defects in the mismatch repair system are associated with a microsatellite unstable phenotype. In this chapter, we describe the preparation of purified plasma cells using CD138 magnetic microbeads as a source of tumor DNA. We also describe a robust, sensitive method for compar- ing microsatellite repeat units of tumor to constitutive DNA using polymerase chain reaction and laser scanning of fluorescently labeled amplicons in an automated sequencer in order to assess microsatellite instability in myeloma.
