This is supported by our findings: (1) circMAST1 is highly expressed in HCC tissues, and HCC cell lines (e.g., HepG2 and HCCLM3); (2) silencing circMAST1 in a murine xenograft model significantly reduces the growth of HCC; (3) circMAST1 is likely required to sustain the cell cycle progression, proliferation, migration, and invasion of HCC cell lines; (4) circMAST1 is a miR-1299 sponge, and silencing circMAST1 inhibits cell growth significantly; (5) circMAST1 sponges miR-1299 to promote CTNND1 expression and is required to sustain cancer progression. Here, CTNND1 is linked to cancer.