There is a direct relationship between the degree of differentiation of cancer cells and their malignancy, and highly differentiated cancer cells are less invasive and less malignant than poorly differentiated cancer cells. Cancer cells are functionally and structurally less differentiated than normal cells. Moreover, the degree of differentiation varies among different cancer cells. If cancer cells are morphologically more similar to normal cells and have more functions of normal cells, the higher their degree of differentiation often represents less invasiveness and malignancy of cancer cells. On the contrary, the more obvious the structural difference between cancer cells and normal cells, and the fewer the functions of normal cells, the lower the degree of differentiation of cancer cells, which often indicates that the degree of malignancy of cancer cells is greater.