SKP2 and osteosarcoma: Skp2 is involved in cell proliferation, apoptosis, migration, invasion, angiogenesis, and metastasis of many malignant human tumors [16,17], and is over-expressed in many types of human cancers, including breast cancer, non-small cell lung cancer [19,20], prostate cancer [21,22], pancreatic cancer [23], multiple myeloma [24], gastric cancer [25,26], melanoma [27,28,29], lymphoma [30,31,32], nasopharyngeal carcinoma [33,34], and osteosarcoma [35,36].