In our study we found that control group had significantly higher MPO serum level after 3 months and after 6 months compared to its baseline, which agrees with Januzzi et al. who found that serum levels of MPO were significantly elevated after 3 months and early changes in MPO were associated with subsequent cardiotoxicity in patients with breast cancer underwent sequential therapy with doxorubicin and trastuzumab [11]. The gene discussed is MPO; the disease is breast cancer.