Rare bloodborne cells like circulating tumor cells are not only challenging to capture given that they are outnumbered by a billion-to-one by normal cells, but their scarcity makes them very difficult to characerize. It is critical to understand the molecular-level properties of CTCs, however, as these cells are the seeds of metastatic tumors and may provide new ways to monitor cancer patients for disease progression and response to therapy. The Kelley laboratories have developed a new method for analyzing CTCs that uses magnetic nanoparticles to target surface proteins on these cells. A microfluidic device captures the CTCs in one of 100 discrete capture zones based on the level of a surface marker present. After a sample is processed, a profile is generated that allows surface expression to be quantitated. This method analyzes CTCs at the single cell level and can track very small collections of cells. Using an animal model of cancer and patient samples, the ability of magentic ranking cytometry to monitor dynamic CTC phenotypes was demonstrated. The paper on this new technique was published in Nature Nanotechnology, along with an excellent News and Views piece from Ian Wong.