EPO and neuroblastoma: Expression of a number of further pro- and anti-angiogenic factors has been suggested to be relevant for neuroblastoma angiogenesis including platelet-derived growth factor α (PDGFα), matrix metalloproteinase 2 (MMP-2), MMP-9, erythropoietin (EPO), EPO receptor, activin A, interleukin-6 (IL-6), leukemia inhibitory factor (LIF), tissue inhibitor of metalloproteinase 2 (TIMP2), pigment epithelial-derived growth factor (PEDGF), secreted protein acidic and rich in cysteine (SPARC), thrombospondin-1, and thrombospondin-2 [33].