Clonal evolution in myeloma. Illustration of the multistep evolution pathway to plasma cell leukemia. Each step in this process seems to be the result of an acquired growth advantage in a subclone that comes to dominate the clinical picture associated with the transformed plasma cell clone. At each stage the genetic complexity of the disease will increase as each of the prior stages of disease should also be detectable if a test with sufficient sensitivity is available. In plasma cell leukemia because of rapid growth and clonal expansion the prior phases may be present at apparently lower levels.