Malignant cells exhibit two constant behavioral features which together determine lethality: cell proliferation and invasion. Since proliferation also produces only benign tumors, invasion is the key to malignancy. All tissue cells are inherently motile, so it is argued that invasion will occur in vivo whenever cell movement is not under control. From embryological studies it is understood that movement is normally controlled by interacting sets of cell surface recognition molecules. Accordingly it is hypothesized that the prime phenotypic defect in malignancy is loss (or masking) of the malignant cells'' recognition molecules. It is believed that malignant transformation occurs in stem cells at critical differentiation "switching" steps involving changes in surface phenotype. A cell recognition defect could arise therefore if cell surface marker deletion is not followed by full expression of a complete set of new markers. Investigations of such a possibility could test the hypothesis.