Figure 2: Application of single-cell sequencing technology in tumor evolution and metastasis. In tumor progression, some tumor cells at the primary site can invade into the basement of blood vessels, migrate into the blood circulation, and then implant in the distant tissue site, defined as metastasis. Circulating tumor cells (CTCs) can be easily collected and single-cell sequencing of CTC can be used to predict tumor metastasis. Determining the driver gene mutations of cellular subpopulations may aid in understanding on the tumor evolution and the development of targeted therapy using chemical drugs and immune cell therapies, such as chimeric antigen receptor modified-T cells (CAR-T cells).