OR51E2 and prostate cancer: Due to their specific expression patterns, some ORs could moreover serve as tumor biomarkers, such as OR51E1 and OR51E2, in prostate cancer (Xu et al., 2000; Neuhaus et al., 2009; Massberg et al., 2016), lung cancer (Giandomenico et al., 2013) and small intestine cancer (Cui et al., 2013) and OR7C1 as a marker of cancer-initiating cells in colorectal cancer (Morita et al., 2016).