CD99 and neuroblastoma: Thus, co-expression of CD56+, CD90+, GD2+, CD81+, and CD9+ in the absence of CD45 is a typical immunophenotypic feature of neuroblastoma (NBL) cells [19,20], while a CD99+ CD45− phenotype has been associated with Ewing sarcoma (ES) [14,18], and co-expression of CD90+, CD56+, and CD57−/+ in the absence of CD45 is most frequently observed in rhabdomyosarcoma (RMS) tumor cells [16,21].