Characterization of circulating tumor cells as a reflection of the tumor heterogeneity: myth or reality?

•Circulating tumor cells are highly heterogeneous in a spatial and temporal manner.•Circulating-tumor cells are a reflection of the tumor tissue at a fixed time.•Circulating-tumor cells exhibit a dynamic phenotype and genomic profile.•Circulating-tumor cells are the photograph of tumor evolution.•Genomic and downstream RNA analyses of CTCs are promising to monitor the therapeutic response.