37,X/38,XY mosaicism in a cat with a female phenotype and an enlarged clitoris.

A 10-month-old mixed-breed cat was presenting a female appearance; however, a detailed examination of external genitalia revealed an enlarged clitoris with spines. The urethra opened into the vagina. Laparotomy and subsequent histopathological examination revealed that both ovaries were replaced by hypoplastic testes, with underdeveloped uteruses located caudal to them. Chromosome and genetic testing using peripheral blood revealed that the cat displayed a nearly 1:1 mosaicism of the 37,X and 38,XY chromosomes and presence of the SRY gene. Only two cases of 37,X/38,XY mosaicism have been reported in cats, both displaying a normal penis and prepuce, indicating a male phenotype. To the best of our knowledge, this is the first report of 37,X and 38,XY mosaicism manifesting as a female phenotype.