Mehmet Toner and his group at the BioMEMS Resource Center, Massachusetts General Hospital describe the illustration that appears on the cover of the current issue of Biophysical Journal. A longstanding interest in our lab is the sorting and separation of complex mixtures of cells, which is important for diagnostics, therapeutics, and basic cell biology. Our general approach has been based on the selective capture of cells using antibody-functionalized surfaces in microfluidic devices. ...