Different types of immune cells in the blood can be recognized for diagnostic and therapeutic purposes by the glycoprotein markers on their surface. For example, B cells typically express the markers CD45 and CD20, whereas T cells have CD45 and CD3. In a new recognition approach—developed by Sergei Rudchenko of the Hospital for Special Surgery in New York City, Milan N. Stojanovic of Columbia University, and coworkers—a set of molecular robots made of double-stranded DNA-antibody conjugates undergo molecular cascades when they bind to particular combinations of cell-surface markers (Nat. Nanotechnol. 2013, DOI: 10.1038/nnano.2013.142). After a combination of robots binds to a cell surface, adding the right single-stranded DNA molecule triggers a cascade of strand displacement reactions. Each reaction starts with an incoming strand binding to a short overhanging sequence and replacing the existing strand to form a new pair of oligonucleotides with a longer complementary sequence than the previous pair. The last reaction in the cascade unveils a molecular label that gives off a fluorescent signal, which is revealed only if the cascade runs to completion. The researchers used the DNA robots to identify groups of cells that express particular combinations of two or three cell markers.