Scientists have discovered how mosquitoes and other insects can smell carbon dioxide, a finding which might help in the discovery of more effective bug repellents. Working initially with fruit flies, New York Rockefeller Institution researcher Leslie Vosshall and her colleagues identified a protein called Gr63a, which pairs up with another protein called Gr21a, and together they form a chemical sensor that allows nerve cells to detect carbon dioxide. And when the researchers added the genes for both proteins to cultures of nerve cells that did not previously express Gr21a or Gr63a, the cells began to respond to CO2. Buoyed-up by the result the team then looked for similar genes in mosquitoes and found the mozzie equivalents, which they've called GPRGR22 and GPRGR24. Mosquitoes use their CO2-sensing abilities to home in on potential victims by following the chemical grail of their breath. So knowing the molecular machinery they are using to do this should help researchers in their search for novel ways to blind mosquitoes to our presence and hence cut down the spread of diseases like malaria, which claims 3 million lives a year.