A typical cancerous tumor contains millions or even billions of cells harboring genetic mutations driving them to grow, divide, and invade the local tissue in which they’re embedded. However, as the cells proliferate, they don’t all stay in the neighborhood. Some cells slough off the edges of a tumor and are swept away by the bloodstream or lymphatic system. These so-called circulating tumor cells (CTCs) can remain loose in circulation, cluster together as they travel, or lodge themselves in new tissues. Whatever their path, their common origin means that CTCs hold information about a tumor, information that researchers think could be key to cancer diagnosis or treatment.