While it would be satisfying to find that the same cellular process, or even a group of closely related processes, leads to the development of neoplasia, there is no necessity to believe that such a common pathway exists, since cancer includes many diverse diseases, the etiology, biology, clinical presentation, therapy, and prognosis of which will differ very greatly. Our knowledge of cancer cells depends largely on the behavior of the cells in culture and the measurement of various parameters that are not necessarily linked in any causal way with the neoplastic state. From these studies, however, have arisen a number of theories on the nature of neoplasia. For example, it is quite clear that the cell surface of cancer cells is abnormal in many different ways.304 There are new antigenic determinants; there may be altered glycoprotein and sialoprotein structures on the cell surface; the electrical charge and therefore the electrophoretic mobility of the cells may be altered; the absence of some material from the cell surface may also be associated with the acquisition of neoplastic properties (e.g., LETS protein29). Cancer cells also have altered contact relationships with other cells in culture in that they will fail to respond to the presence of other cells and move over or under them, whereas normal cells will usually stop or change direction on coming into contact with other cells.