SFRP4 and head and neck squamous cell carcinoma: In addition, several other studies have shown that numerous canonical Wnt signal pathway inhibitors, including SFRP4, all-trans retinoic acid (Atra), and active natural compounds and honokiol, can reduce the expression of β-catenin and ultimately inhibit the proliferation of CSC in HNSCC (Lim et al., 2012; Yao et al., 2017).