In patients with acute leukemia, treatment decisions are based on the status of peripheral blood and bone marrow cellularity. This provides a measure of the efficacy of therapy and can reveal leukemia relapse. The reliability of morphologic examination of peripheral blood and bone marrow largely depends on the hematologist's expertise, and its sensitivity is fundamentally limited by the similarities in appearance between leukemic cells and normal lympho-hematopoietic progenitors. Therefore, patients in complete morphologic remission may still have a large number of residual leukemic cells (potentially up to 1010).