AbstractINTRODUCTION Cells cultured from animal and human tumors can be induced to differentiate by over 300 agents, including powerful nonphysiologic inducers such as polar solvents and Na-butyrate, and physiologic inducers such as hormones, regulatory peptides, retinoids, vitamin D3, proteases, hemin, and hypoxanthine (for review see Refs. 1,2). Differentiation can be defined as a coordinated genomic expression whose products are characteristic of a more defined cell lineage (e.g., differentiation of erythroleukemia cells is characterized by globin gene expression, followed by hemoglobin production). When differentiation of tumor cells is induced, it is often followed by reduced tumorigenicity due to a decrease in the proliferation rate. Agents that induce differentiation in leukemia, embryonal carcinoma, mammary carcinoma, and mouse melanoma cells in vitro, can also reduce tumor growth in vivo (for review see Refe 2,3). Anticancer treatments such as radiation, cytotoxic chemotherapeutic drugs, or interferons, possess in vitro differentiation-inducing capabilities in cells derived from a wide variety of neoplasms (for review see Refs. 4,5). Recent studies have shown that differentiation of cancer cells by certain cytotoxic chemotherapeutic drugs may be part of the mechanism involved in the clinical response (6). The concept of differentiation by cytotoxic agents, such as low concentration (10−8M) cytosine arabinoside, has been utilized in the treatment of acute leukemias and myelodysplastic syndromes in elderly patients (7,8). Such differentiation chemotherapy would differ from standard cytocidal chemotherapy in that sublethal doses may still be effective. In addition, analogs of cytotoxic agents with enhanced ability to induce differentiation, but with reduced cytotoxicity, can be sought.