Circulating breast cancer cells. Fluorescence light micrograph of a cluster of circulating tumour cells (CTCs, red) and white blood cells (green) from the blood of a patient with breast cancer. CTCs are an important focus of cancer research because they provide a key target for new treatments. By identifying the molecular adaptations and genetic mutations that allow CTCs to spread (metastasize), it may be possible to develop strategies to block metastatic cancer on a patient-by-patient basis.