ABCG2 and retinoblastoma: Although it has been shown in a mouse model that vascular endothelial progenitor cells with the overexpression of ABCG2 have a higher potential for vessel formation in vivo compared to mature endothelial cells, no association has been demonstrated between ABCG2 gene expression and VEGF gene expression and tumor vascularization in patients with retinoblastoma [75,76].