PLK1 and cancer: Shikonin and its analogues could kill cancer cells via inhibiting topoisomerase-I [7], [8], [9], polo-like kinase 1 (PLK1) and protein tyrosine kinase (PTK) [10], [11], regulating the activities of phosphorylated extracellular regulated protein kinase (pERK), c-Jun N-terminal kinase (JNK), and protein kinase C-a (PKC-a) [12], suppressing the expression of tumor necrosis factor receptor-associated protein 1 (TRAP1) [13], activating caspase activities[14], [15], inhibiting proteasome [16], among others.