Granulocyte and monocyte identification and phenotyping relies more heavily on morphology than is possible with lymphocytes. Light microscopic appearance can, if necessary, be supplemented by a limited number of cytochemical stains and enzyme tests, and in some cases by immunologic tests for CD antigens. In some cases phenotyping may require chromosome analysis using standard methods (e.g., the Philadelphia chromosome in chronic myelogenous leukemia [CML]), nucleic acid probe methods (e.g., breakpoint cluster region (BCR) gene rearrangement in CML), or immunologic tests for certain CD antigens. The morphologic and cytochemical approach is best seen in the French-American-British (FAB) classification of acute leukemias.