PLK1 and breast cancer: Recently, PLK1 has been found to be overexpressed in many types of human cancers such as hepatocellular carcinoma [24–28], glioma [29–32], colorectal cancer [33–38], gastric cancer [39–42], pancreatic cancer [43–45], lymphomas [46,47], ovarian cancer [48,49], bladder cancer [50], prostate cancer [51], breast cancer [52,53], and esophageal cancer [54].