Setting out to isolate circulating tumor cells (CTCs) in the blood of cancer patients, Cima et al.instead noticed that clusters, rather than single cells, abounded. But these clusters were not like CTCs: The cells did not express the epithelial cell marker EpCAM and they did not have the same mutations as the primary tumor. Instead, the clusters matched the genotype and phenotype of endothelial cells and were shed from tumors in mice and in humans as intact clusters, rather than forming by coagulation. In patients, the presence of endothelial cell clusters correlated with early-stage disease and might be a unique indicator of cancer before treatment starts.
