Multiple myeloma is a cancer that is found in the plasma cells which are located in bone marrow. As with all cancers, the normal healthy cells mutate into cancer cells and multiply at a faster rate than a healthy blood cell. When these cells start multiplying at a faster rate, they produce a tumor in the bone marrow called a plasmacytoma. If just one tumor is detected in the bone marrow it is called an isolated plasmacytoma. When it is more than one tumor, it is classified as multiple myeloma.