PTPN22 and cancer: Actually, several early-discovered peptides may have the tumor-penetrating activity similar to iRGD because they contain a potential CendR motif, such as F3 (a 31 amino acid peptide homing to cancer cells and tumor endothelial cells) [65], LyP-1(CGNKRTRGC), homing to cancer cells, tumor macrophages and lymphatic vessels [66], CREKA and CSRPRRSEC (two peptides homing to tumor blood vessels) [58,67].