P2RX4 and transitional cell carcinoma: We used TRT-HU-1 cells, an hTERT-immortalized and non-transformed human urothelial cell line [33], RT4 cells, a low-grade, well-differentiated, non-invasive papillary transitional cell carcinoma cell line, and T24 cells, a transitional cell carcinoma cell line that was established from a bladder cancer patient with a high-grade, invasive urothelial carcinoma and that expresses various purinergic receptor subtypes [34,35].