ARHGEF15 and mesenchymal cell neoplasm: Mesenchymal tumors are quite common in cattle harboring papillomavirus infection and grazing on lands rich in bracken fern, which shows that E5 protein, the major oncoprotein of BPV-2 and ptaquiloside and the major oncogenic factor of bracken fern, may play a synergistic role also in mesenchymal cancerogenesis of the urinary bladder.