Schematic diagram of the proposed mechanism for COL4A1 and COL13A1 in human urothelial carcinoma of the bladder. COL4A1 (green circle) and COL13A1 (red circle) are localized to the stromal area around the tumor site and the cytoplasm of tumor cells, respectively. Overexpression of these two subunits of collagen in the tumor lesion leads to tumor invasion, progression, and eventually metastasis. EMT = epithelial-mesenchymal transition.