Preclinical data provide support for pursuing Plk1 as a target for cancer therapy, suggest the potential advantage of Plk1 selective compounds, and provide early evidence for potential biomarkers for patient selection (TP53 and/ or RAS mutations). Both preclinical and clinical data pro- vide support for the use of pHH3 as a PD biomarker for Plk1 inhibition. As expected for a compound with antimi- totic activity, hematologic toxicities were observed for most Plk inhibitors evaluated in clinical studies. Clinical activity has been noted following treatment with Plk1 in- hibitors, however, a narrow therapeutic index has been observed with activity occurring at or above the MTD. In summary, Plk1 seems to be an appealing target for cancer therapy, however, further evaluation is necessary to dis- cern the utility of Plk1 inhibitors in a clinical setting and determine how they will compare with currently available antimitotic agents.