ERBB2 and breast cancer: The effectiveness of trastuzumab as adjuvant treatment for HER2-positive breast cancer has also been studied in observational studies (Zurawska et al. 2013; Webster et al. 2012; Peterson et al. 2014; Palmieri et al. 2011; Jensen et al. 2012; Inwald et al. 2014; Hayashi et al. 2013; Bonifazi et al. 2014; Vici et al. 2014).