‘Gain of function’ is a chilling new expression—well, new to most of us—that has entered our language as a result of investigations into where the original coronavirus came from: a wet market or a lab leak? But where does this shiver-down-the-spine expression ‘gain of function’ come from? And what does it mean? The expression seems to go back to 2011 when two groups in different universities were experimenting on bird flu (specifically H5N1) to find out if they could force it to change its host population from birds to mammals.