SFRP4 and head and neck squamous cell carcinoma: Furthermore, several other studies have also demonstrated a reduction in expression of β-catenin and eventually a suppression in the proliferation of CSC in HNSCC by several inhibitors of canonical Wnt signaling pathway including secreted frizzled-related protein 4 (sFRP4), all-trans-retinoic acid (ATRA) and honokiol, an active natural compound (Lim et al., 2012; Yao et al., 2013; Warrier et al., 2014).