CXCR4 and cancer: Furthermore, a CXCR4 antagonist peptide called EPI-X4, which was identified as a proteolytic fragment from an albumin precursor protein, inhibits cancer cell migration and stimulates the migration of hematopoietic stem cells.19 The nature-derived peptide motixafortide, isolated from a horseshoe crab protein, is a 14-mer cyclic CXCR4 antagonist (BL-8040).20 It is being investigated in multiple clinical trials for the treatment of leukemia and solid tumors.