HSPG2 and astrocytoma (excluding glioblastoma): Potent P2Y6R agonist 5-iodouridine-5′-O-diphosphate (MRS2693, EC50 15 nM in PLC activation) and three antagonists were tested for effects on cell viability using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay [44] in two cell lines, hP2Y6R-1321N1 human astrocytoma cells and HeLa cells (Figure 5, Table S3).