Since RANKL/RANK/OPG are regulated via sex hormones, explaining, for instance, how progesterone can drive breast cancer (Schramek et al. 2010; Nolan et al. 2016; Sigl et al. 2016) and how sex hormones influence bone metabolism (Manolagas et al. 2014), we speculated that this system might also be involved in the long speculated and proposed gender differences in lung cancer (Chlebowski et al. 2009), lung cancer having become the number one “killer” for women with cancer (Siegel et al. 2015). The gene discussed is TNFSF11; the disease is breast cancer.