The program for apoptotic cell death activated in the absence of hematopoietic factors is present not only in normal hematopoietic progenitor cells but throughout the differentiation program to mature neutrophils, eosinophils and macrophages (reviewed in refs. 33 and 81). Viability factor withdrawal also induces apoptosis in hematopoietic factor-dependent cell lines that are no longer normal but not yet malignant.