The impaired NK-cell-mediated degranulation was due to a dual mechanism of innate immune evasion: (i) the strong upregulation of MHC class I, which is known to cause an imbalance of activating versus inhibitory signals on NK cells [75] accompanied by downregulation of activating ligands at different levels; and (ii) the strong upregulation of IDO1, a molecule involved in most of the immune evasion mechanisms of tumors [66], including NB [67]. Here, IDO1 is linked to neuroblastoma.