PLK1 and breast cancer: Results of the meta-analysis indicated that high PLK1 expression significantly correlated with large tumor size (tumor size > 2 cm) with low heterogeneity (OR = 1.703, 95% CIs: 1.315-2.205, P < 0.001; I2 = 30.10%, P = 0.198, Figure 2a) in six studies with 1779 BC patients; higher tumor grade (OR = 6.028, 95% CIs: 2.639-13.772, P < 0.001, Figure 2b) in 1989 BC patients; and lymph node metastasis (OR = 1.524, 95% CIs: 1.192-1.950, P = 0.001, Figure 2c) in 1975 BC patients.