PECAM1 and cancer: Besides previously reported clinical utilities of co-detection of aneuploid CD31<sup>-</sup> CTCs and CD31<sup>+</sup> CTECs in real-time evaluation of therapeutic efficacy, longitudinal monitoring of emerging treatment resistance and adequate detection of MRD, a large cohort study is necessary to further investigate whether, and how, a high ratio of MRD CTECs to CTCs may function as an appropriate index forecasting either occurrence or metastatic distant recurrence of malignancy in post-therapeutic cancer patients.