A particular differentiation product of urothelial cells,uroplakins (UPs) are a family of transmembrane proteins(UPs Ia, Ib, II, and III). UPs are expressed in the luminalsurface plasmalemma of superficial (umbrella) cells in thenon-neoplastic mammalian urothelium, where they formcomplexes of crystalline particles measuring 16 nm. Whencombined with additional indicators, UPIII, which is specificfor urothelial tumours, can help diagnose both primary andmetastatic tumours.