Di Oto E, et al. - The role of the X chromosome gain in the development of male breast cancer (MBC) and its relation with androgen receptor (AR) gene copy number and expression were investigated in this analysis. For the purpose of this research, the X chromosome status was evaluated in 66 cases of male invasive and in situ duct breast carcinoma, in 34 cases of gynecomastia associated with cancer, and in 11 cases of tumor-free gynecomastia. Results of this study suggested that X chromosome could play a role in the neoplastic transformation of male breast epithelium. It was noted that X chromosome gain was paralleled by AR gene polysomy. On immunohistochemistry (IHC), polysomic AR genes showed low methylation levels and high AR protein expression. For MBC treatment planning, these data should be taken into consideration.