The study of the phenotypic and genetic features that characterize the malignant cell is a rapidly growing and changing field. Clearly new insights into the processes involved in normal and abnormal cell growth will facilitate our understanding of events relevant to cancer and cellular differentiation. Early studies on genetic fea­ tures associated with cancer focused on chromosomal abnormalities that were observable in several human malignancies. The more recent examination of onco­ genes and the proteins they encode has helped pinpoint many steps in different processes that might be involved in cancer. Immunologic studies of cancer have also developed from an imprecise series of investigations to a more detailed molecular examination of cell-surface struc­ tures that can be recognized immunologically. In the course of the development of modern tumor immunology, it has become clear that many of the antigens that can be recognized appear to be the products of genes involved in cell growth. Fur­ thermore, changes in the cell surface of malignant cells have often been found to include alteration of nonprotein constituents.