In addition to certain steroids, chemotherapeutic compounds, and radiation, other compounds that can induce differentiation in myeloid leukemic cells include insulin, bacterial lipopolysaccharide, certain plant lectins, tumor promoting phorbol esters (reviewed in refs. 53 and 54), and retinoic acid (reviewed in ref. 75). This effect of retinoic acid on differentiation of promyelocytic leukemia cells is now used clinically in the therapy of these leukemias (75), showing the successful application of the concept of differentiation therapy in the clinic. It is possible that all myeloid leukemic cells that are no longer susceptible to the normal hematopoietic cytokines by themselves can be induced to differentiate by the appropriate combination of compounds. The experiments with myeloid leukemic cells have shown that there are different pathways of gene expression for inducing differentiation, and that genetic changes which suppress induction of differentiation by one compound need not affect differentiation by another compound by alternative pathways (refs. 76 and 77; reviewed in refs. 25 and 54). These results show that there is considerable flexibility in the myeloid differentiation program, and this presumably also applies to other cell types.