Identification of normal growth and differentiation-inducing proteins and how they interact in normal development has made it possible to identify the molecular basis of normal development and the mechanisms that uncouple growth and differentiation so as to produce malignant cells. When normal cells have been changed into cancer cells, the malignant phenotype can again be suppressed. Results on the molecular control of growth and differentiation in normal myeloid hematopoietic cells, changes in the normal developmental program in myeloid leukemia, and the suppression of malignancy in myeloid leukemia and sarcomas have shown that (1) malignancy can be suppressed either with or without genetic changes in the malignant cells, (2) suppression of malignancy by inducing differentiation does not have to restore all the normal controls, and (3) genetic abnormalities which give rise to malignancy can be bypassed and their effects nullified by inducing differentiation which stops cells from multiplying.