COX-2 expression in multiple cells in tumors of different stages. The sequence of events shown has been constructed from studies with multiple types of cancer. For example, the work on the earliest events was from mouse and human experiments on adenomatous polyps of the colon, in which the earliest expression of COX-2 (shown in red) is in the stromal cells. In several types of cancers, COX-2 is found in multiple cells in the late stages. Metastatic tumors in bone and various other organs express COX-2 in the metastatic cells and surrounding cells. In the case of bone, osteoclasts, which degrade the bone matrix and allow growth of the newly metastasized tumor, are among these COX-2⁺ cells.