Cancer is a leading cause of death in cats. Epithelial tumors in the mammary gland, oral cavity, and skin are the most common types of cancer, with most being malignant (carcinomas). Currently, we rely on invasive biopsy procedures for tumor diagnosis and prognostication. There is tremendous interest in identifying novel blood biomarkers that can be used for tumor detection and prognostication – so-called “liquid biopsies”. One such biomarker is the presence of circulating tumor cells (CTCs). These cells shed from the primary tumor are the precursors of metastatic lesions. CTCs can be detected and quantified in the blood of humans with carcinomas, based on expression of surface antigens or mRNA profiles that are characteristic of the primary tumor.