The term "on paper" often means "theoretically", but James Collins of Harvard University, Boston University, and the Howard Hughes Medical Institute in Maryland and colleagues have shown that working biological circuits can be printed on paper. They produced cell-free gene networks made from off-the-shelf parts and freeze-dried onto paper. When rehydrated, these worked as diagnostic devices with a colorimetric output, capable of detecting glucose and distinguishing RNA fragments from two related species of the Ebola virus.