Besides its homophilic interaction [26], the existence of other CD99 ligands has been proposed [45, 46, 60]; such as the GDF6 prodomain, which maintains Ewing sarcoma growth [61] or the paired immunoglobulin-like receptors (PILRs), capable of binding mouse CD99 [62, 63]. This evidence concerns the gene GDF6 and Ewing sarcoma.