Whereas the silencing of endogenous CD99 revealed a tumor-promoting role in Ewing sarcoma (Kreppel et al., 2006; Rocchi et al., 2010), exogenous expression of CD99 uncovered a tumor-suppressing role in osteosarcoma (Manara et al., 2006; Sciandra et al., 2014; Scotlandi et al., 2007; Zucchini et al., 2014). Here, CD99 is linked to Ewing sarcoma.