RSPO1 and cancer: Lgr5 is specifically expressed on cancer stem cells (CSCs) and is known to amplify the effect of Wnt/β-catenin signaling by working as a receptor for R-spondins (RSPO), which are well established and influential agonists of the Wnt pathway (Carmon et al., 2012), and known key drivers of oncogenesis (de Lau et al., 2014).