No, cats are a bit different from other mammals: In most mammals, including humans, red hair is caused by mutations in one cell surface protein, Mc1r, that d

No, cats are a bit different from other mammals: In most mammals, including humans, red hair is caused by mutations in one cell surface protein, Mc1r, that determines whether skin cells called melanocytes produce a dark pigment or a lighter red-yellow pigment in skin or hair. Mutations that make Mc1r less active cause melanocytes to get “stuck” producing the light pigment. But the gene encoding Mc1r didn’t seem explain where cats’ orange fur came from. It isn’t located on the X chromosome in cats or any other species—and most orange cats don’t have Mc1r mutations. “It’s been a genetic mystery, a conundrum,” says Greg Barsh, a geneticist at Stanford University.