Cancer is a complicated disease caused by genetic variation. The genetic variation may deprive cells of the normal control function, leading to immortalization and cancer. And targeted drugs work well in cancer therapy, which can kill cancer cells that carry specific genes of variation. However, patients may differ from each other regarding the in vivo genetic variation. As a result, patients vary in the susceptibility and toxic and side effects to the same anti-cancer drugs. As the more inclusive “molecular typing” is proposed, more patients are able to accept targeted therapy, where physicians can custom therapeutic regimens for cancer patients varying in genetic variation to tap as many potentially available targeted drugs as possible and enhance therapeutic efficiency of the drugs. That’s how precision medicine works. Precision medicine needs a prerequisite - precision diagnosis.