Abstract Aim: We hypothesized that patients with Klinefelter’s syndrome (KS) not only undergo X inactivation, but also that genes escape from inactivation. Their transcripts would constitute a significant difference, as male metabolism is not adapted to a ‘female-like’ gene dosage. We evaluated the expression of selected X-linked genes in our 41, XXY* male mice to determine whether these genes escape inactivation and whether tissue-specific differences occur. Methods: Correct X inactivation was identified by Xist expression. Relative expression of X-linked genes was examined in liver, kidney and brain tissue by real-time PCR in adult XXY* and XY* males and XX females. Results: Expression of genes known to escape X inactivation was analysed. Relative mRNA levels of Pgk1 (control, X inactivated), and the genes Eif2s3x, Kdm5c, Ddx3x and Kdm6a escaping from X inactivation were quantified from liver, kidney and brain. Pgk1 mRNA expression showed no difference, confirming correct X inactivation. In kidney and liver, XXY* males resembled the female expression pattern in all four candidate genes and were distinguishable from XY* males. Contrastingly, in brain tissue XXY* males expressed all four genes higher than male and female controls. Conclusion: Altered expression of genes escaping X inactivation probably contributes directly to the XXY* phenotype.