Thus, in 5/5 infiltrated EES samples, tumor cells displayed a CD99+ CD9+ nuMyoD1− phenotype with expression of GD2lo in 2/5 (40%), CD117+ in 2/5 (40%), CD90+ in 4/5 (80%), CD57+ CD10+ in 1/5 samples (20%), CD105+ in 2/2 (100%) of samples tested and dim expression of numyogenin in association with coexisting EWS and PAX3 gene rearrangements in 1/5 EES tumors. This evidence concerns the gene CD99 and neoplasm.