Catherine Alix-Panabieres, PhD, University Medical Center of Montpellier, Montpellier, France, discusses research into circulating tumor cell (CTC) lines in colon cancer. Research into a small, aggressive subset of CTCs which are at the origin of metastatic relapse aims to develop effective treatments against them. There are several challenges to overcome, including the resistance mechanisms that the CTCs can develop against the drugs, as well as the high plasticity of the CTCs. A highlight of this research is that the origin of some of these CTCs is the liver, which is in new in this field. Additionally, Prof. Panabieres discusses the importance of researching PD-L1-expressing CTCs in breast cancer and non-small cell lung cancer. This interview took place at the International Society of Liquid Biopsy (ISLB) 2021 Virtual Meeting.