PVR and breast cancer: Recent studies have found that CD155 is expressed at low levels in many normal cells, such as epithelial cells, endothelial cells, nerve cells, and fibroblasts (Yong et al., 2019), but is expressed at high levels in many tumors, such as colorectal cancer (Masson et al., 2001), gastric cancer (He et al., 2017), ovarian cancer (Carlsten et al., 2009), melanoma (Inozume et al., 2016), lung cancer (Sun et al., 2020), and breast cancer (Triki et al., 2019).