Rigosertib potassium (formerly also known as ON-01910), the sodium salt of Rigosertib, is a novel, potent and non-ATP-competitive inhibitor of PLK1 with IC50 of 9 nM in a cell-free assay. It shows 30-fold greater selectivity against Plk2 and no activity to Plk3. Rigosertib can be potentially used for the treatment of cancer. Rigosertib inhibits polo-like kinase1 (Plk1), inducing selective G2/M arrest followed by apoptosis in a variety of tumor cells while causing reversible cell arrest at the G1 and G2 stage without apoptosis in normal cells. This agent may exhibit synergistic antitumor activity in combination with other chemotherapeutic agents.