A mutation that protects against AIDS otherwise shortens lives

IT IS A good rule in matters genetic to assume that things are there for a purpose. This is not to say that the wrong sort of DNA cannot cause disease. It can. But disease-causing mutations are rare, because natural selection makes them so. Far more common are genes of pedestrian function that are somehow dismissed as dispensable.One such is CCR5, which encodes a protein found on the surface membranes of certain types of cells, especially cells of the immune system. CCR5 is one of a group of membrane proteins called beta chemokine receptors. These are involved in inflammatory responses, but CCR5 does not seem to be essential to such responses. What is crucial about it, though, is that in the wrong circumstances it can harm the body it is in. The reason is that it is one of the proteins onto which HIV, the virus that causes AIDS, latches itself before entering a cell and infecting it.