Castriconi, Roberta, et al. “PD-L1 Expression in Metastatic Neuroblastoma As an Additional Mechanism for Limiting Immune Surveillance”. Italian Journal of Anatomy and Embryology, vol. 120, no. 1, Sept. 2015, p. 26, https://oajournals.fupress.net/index.php/ijae/article/view/3980.