Multiple myeloma is a hematological malignancy characterized by an increased number of bone marrow plasma cells and the presence of high levels of a serum monoclonal immunoglobulin that is coded by a unique genetic sequence in the variable region of the immunoglobulin heavy chain gene. Morphological examination of bone marrow biopsy samples prior to therapy and serum immunofixation studies of the monoclonal immunoglobulin are usually sufficient to clearly diagnose this disease. However, after therapy when the number of malignant cells is reduced, it is not possible to differentiate between the small number of malignant and normal (polyclonal) plasma cells using traditional microscopy. In addition, while the malignant cells are generally found predominantly in the bone marrow, in many patients with progressive disease the malignant cells may spill over into the peripheral blood.