Klinefelter syndrome has been described in various species in addition to humans, including cat, pig, horse, and dog. It is associated with low levels of male hormones, sterility, breast enlargement, and small testes. Patients with Klinefelter syndrome have a higher risk for several malignancies. Knowledge about genetic disorders of the dog is comparatively sparse. This is mainly due to the difficult canine karyotypic pattern. We present the case of a canine patient with clinically and cytogenetically confirmed Klinefelter syndrome who developed a testicular tumor at a very early age. Testicular tumors are common in dogs, normally affecting elderly patients (median age, >9 years). In the present case, however, the dog was only 5 years old, allowing the conclusion that the XXY constitution may have promoted the early onset of testicular tumor disease.
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