Those peptides are often based on naturally occurring ligands, e.g. the 17 amino acid CXCL12 analogue CTCE-990827, and the two cyclic peptides LY251092415 and BKT-14028 that are currently tested in phase II clinical trials determining their effect on different kinds of cancer and stem cell mobilization20,29 (for a review see30). Here, CXCL12 is linked to cancer.