tl;dr: Two independent research teams identified a 5-kilobase deletion in a non-coding region near the Arhgap36 gene on the X chromosome as the cause of orange fur in cats, solving a 60-year genetics mystery. The deletion causes the gene to be overexpressed 13x in melanocytes, suppressing dark eumelanin pigments in favor of reddish pheomelanin. Because the mutation is X-linked, most orange cats are male, while females typically display calico or tortoiseshell patterns due to random X-inactivation.