GDF6 and Ewing sarcoma: Growth and differentiation factor 6 (GDF6) in Ewing sarcoma, paired immunoglobulin-like type 2 receptors (PILRs), CD99 antigen-like protein 2 (CD99L2), and CD99 itself were shown to act as ligands of CD99 and contribute to the regulation of the immune system [5,6,7].