The tortoiseshell coat colour of the domestic cat is produced by the epistatic effect of the Sex-linked orange gene (O) on the autosomal black or tabby genes, In cells with an Q gene on the X chromosome, the fur will be L Orange (ginger), even if there are black or tabby genes on the autosomes. Therefore male cats can be either (XY)-Y-O ginger toms, or X-Y black or tabby toms, depending upon which autosomal genes are present. Females with two X chromosomes have three possibilities: X-X- cats will be black or tabby females: depending upon which autosomal genes are present; (XXO)-X-O animals will be ginger females, and (X-XO) Cats will be tortoiseshell females. The variegated pattern of the tortoiseshell coat colour is produced by the random inactivation of one of the X chromosomes in each XX cell, which in the cat occurs on about the 12th day of embryogenesis. In theory, male tortoiseshell cats should not occur, because normal males do not have two X chromosomes, and thus the two coat colours (orange and non-orange) cannot occur in the same: animal. Nevertheless, many male tortoiseshell cats have been reported and most have been found to have an abnormal chromosome complement. In this article the chromosome complement and histological analysis of tortoiseshell males in the studied cat population of the United Kingdom and USA have been described. With an estimated 7.2 milion cats in the UK, and a prevalence of 0.43 % of tortoiseshell male cats being chromosomally abnormal(in the USA - 0.033%), approximately 14 700 cats could possess a chromosome complement of (XY)-Y-O/X-Y, XX/XY or XXY.