Provided is a cyclic peptide which comprises:(i) a CDK4 peptide region； and(ii) a cell-penetrating region；wherein the CDK4 peptide region comprises the amino acid sequence P1R1x1y1R2P2V (SEQ ID NO: 1), in which P1 and P2 are each proline, R1 and R2 are each arginine and each of x1 and y1 are either a linker or proline, wherein if x1 is a linker then y1 is proline or if x1 is proline then y1 is a linker, or wherein x1 and y1 when taken together form a linker, and wherein V may be present or absent； andwherein the cell-penetrating region is capable of enhancing the uptake of the cyclic peptide or a part thereof into cancer cells and comprises an amphiphilic amino acid sequence； and wherein the cyclic peptide or a part thereof is cytotoxic to and/or inhibiting to the growth of a cancer cell.
