The three major characteristics of cancer cells are unrestricted proliferation, loss of contact inhibition phenomenon and distant metastasis. The so-called unrestricted proliferation is often called immortalization. All normal living cells in human body have a life span and will age and die when they reach a certain life span, while cancer cells can grow without restriction without aging and death because of the problems of genes regulating aging and death. Contact inhibition is a characteristic of normal tissue cells, which means that cells in tissues will stop growing and remain quiescent when they reach a certain number and come into contact with each other, so that they can maintain normal tissue and organ size. Cancer cells lose contact inhibition and do not stop growing even though they appear to be crowded one by one. Distant metastasis is better understood as the ability of cancer cells to leave their original growth site and move to other parts of the body to continue to grow, a characteristic that is not present in normal tissue cells. In fact, these three characteristics of cancer cells are complementary and causal. Cancer cells lose the contact inhibition feature because they never age and die, while distant metastasis is the inevitable result of no contact inhibition, because cancer cells without contact inhibition will cause cancer tissues to grow indefinitely, and when the original location cannot accommodate them, they will inevitably find other locations to continue to grow.