Researchers, pet owners and cat chow manufacturers have long recognized that cats, in stark contrast to their canine counterparts, show no particular attraction to sugar. Having sampled two dishes of water, one spiked with sugar and the other not, a cat is as likely to lap from one as the other. But why? The sweetness receptor in mammals consists of two different proteins attached to each other on the surface of a cell. The two are manufactured under the direction of two genes, then embrace to make a single receptor that fires a nerve signal to the brain when sugar is present. In cats, one of those genes, called Tas1r2, is missing a stretch of 247 bases -- a deletion that prevents the gene from making a proper protein. With only one of the two crucial proteins, the cats have no way to taste sweetness.