Cancer genomic medicine or cancer immunotherapy has led to a paradigm shift in cancer treatment. When the first treatment does not work, patients may be able to have second-line therapy or additional rounds of treatment after that, however, most advanced cancers eventually acquire resistance to those treatments. To stop this perpetual cycle, a deeper understanding of cancer evolutionary trajectories during the acquisition of therapeutic resistance is needed. We and others have recently provided evidence that non-genetic drug resistance is due to dormant persister cells, yet little is known about how persister cancer cells promote tumor relapse. To study the non-genetic evolution of cancer cells, a single-cell analysis will enable us to trace the phenotypic plasticity of cancer cells. As persister cancer cells are considered to act as a reservoir for drug-resistant mutants, we may be able to overcome cancer relapse or metastasis if we can better understand their evolutionary trajectories.