An example of a solitary dormant cancer cell detected in a secondary site (liver) 25 days after injection of a tumorigenic but poorly metastatic mammary carcinoma cell line into the mammary fat pad of a mouse. The cancer cell can be recognized by its large size and altered nuclear morphology, relative to the surrounding hepatocytes. However, such solitary cells are difficult to detect in histological sections. Using a fluorescent labeling procedure that permits detection of cells that have not divided, large numbers of solitary, dormant cancer cells were detected and quantified in secondary sites, from both poorly and highly metastatic mammary carcinoma cell lines, and were found to be negative for markers of proliferation and apoptosis. Such solitary cells may contribute to tumor dormancy and recurrence and would not be affected by therapies that target dividing cells. For details, see the article by Naumov et al. on page 2162 of this issue.