ERBB2 and metastatic prostate carcinoma: While EGFR and HER2 levels in these samples can come from anywhere in the body, multiple studies in both animal models and in patients with various types of cancers have demonstrated that in patients with cancer, serum HER2 levels accurately reflect the intratumoral levels of this protein.43–45 In metastatic prostate cancer, circulating levels of HER2 have often been used as a predictive marker of progression.46–49 Therefore, we are assured that the effects of lapatinib observed in the serum also reflect the effects of lapatinib occurring in the tumour tissue.