CD81 and neuroblastoma: Although PNET tumors showed a phenotype which resembled that of neuroblastoma (CD45−, CD56hi, CD90+, CD9+, CD81+, nuMYOD1−, numyogenin− in the absence of B-, T- and myeloid-associated markers), they were negative for GD2, except for one with low expression on a small tumor population 48%, and showed stronger expression of CD99hi and CD271hi (Table 2; Figures 1 and 2).