Using publicly available datasets on single-cell RNA sequencing of Parsotix-filtered CTCs from the blood of patients with breast cancer (Gkountela et al., 2019; Aceto et al., 2014), we also found elevated CD81 expression in CTC clusters compared to single CTCs, confirming the clinical relevance of CD81 as a possible biomarker in CTC clusters and breast cancer metastasis (Figure 4—figure supplement 3D). This evidence concerns the gene CD81 and breast cancer.