TCS HDAC6 20b is an inhibitor of histone deacetylase 6 (HDAC6) that inhibits the growth of HCT116 cells synergistically when used at a concentration of 5 µM in combination with paclitaxel , but it has no activity on cell growth in HCT116 cells when used alone. It inhibits cell growth of α-positive MCF-7 breast cancer cells treated with estrogen but not untreated cells.