HDAC4 and mesenchymal cell neoplasm: In addition to its correlated pathway activation in mesenchymal tumors, HDAC4 overexpression led to increased cell proliferation, decreased reactive oxygen species (ROS) production, and increased invasiveness of U251 cells in vitro, and HDAC4 knockdown in U87 cells in vitro induced the expression of p21WAF1/Cip1, a cyclin-dependent kinase inhibitor and tumor suppressor involved in cell cycle regulation (55, 57, 58).