POU5F1 and bladder transitional cell carcinoma: TM4SF1 marks bladder progenitor cells: TM4SF1, a member of the tetraspanin family reported to be a tumor‐specific antigen, promotes proliferation, invasion, EMT, and chemo‐resistance.[37] The expression of TM4SF1 marks stem cells in mesenchymal tissue[38] and lung.[39] Previously, we identified TM4SF1 expression as a marker of aggressive high‐grade BLCA.[40]TM4SF1 is co‐expressed with OCT4, a well‐known pluripotency marker, in the normal urothelium.