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This work aimed at giving a deeper insight into peculiar cases of intersexuality occurring in dogs and known as XX true hermaphrodism due to the existence of both testicular and ovarian tissue in one or both gonads in the presence of an XX chromosome constitution. Clinical, histological and genetic approaches were used in the study of an 8-month-old Cocker Spaniel dog and a 3-year-old mixed-breed Pitbull, both showing a female phenotype, clitoromegaly and male behavior. A normal female karyotype (2n = 78,XX) was noticed, and polymerase chain reaction failed to detect SRY in genomic DNA obtained from peripheral blood lymphocytes of both dogs. The reproductive tract was removed by standard ovariohysterectomy and processed for histology. Thereafter, a normal female phenotype was reconstructed by vaginoplasty. Histological examination revealed bilateral ovotestis in both cases: the gonads showed immature testicular parenchyma containing seminiferous tubules, Sertoli and Leydig cells, but no signs of spermatogenesis, together with differently developed ovarian follicles containing oocytes. In the ovotestes, steroidogenesis was detected by P450c17-immunoreactivity in Leydig cells as well as in theca cells, whereas no MIS-immunoreactivity was shown by the Sertoli cells. Genital tracts of Wolffian and Müllerian origin co-existed in both subjects. Both dogs belong to the very rare cases in which testicular tissue develops in the absence of the key gene, SRY. Up to date very few genetic events have been associated with this abnormal sexual differentiation: SOX9 over-expression and RSPO1 mutation. Nevertheless, neither of them has been found in these dogs. Copyright © 2011 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]