qRT-PCR analysis revealed that PLK1 mRNA levels were higher in clinical osteosarcoma tissues (n = 20, 18/20 primary, 1/20 lung metastases, and 1/20 case breast metastases) than that in adjacent normal tissues (Fig. 2A), in line with the result by analyzing the data from osteosarcoma patients in the TARGET database (Fig. 2B). Here, PLK1 is linked to osteosarcoma.