TF and breast cancer: Using the IVW method, we found no evidence of heterogeneity for the associations between the status of the four iron markers (iron, transferrin, ferritin and transferrin saturation) and breast cancer risk (for overall breast cancer: Q 0.02, 7.59, 1.12 and 0.08; for ER-positive breast cancer: Q 4.99, 6.21, 2.07 and 3.16; for ER-negative breast cancer: Q 3.65, 7.68, 2.81 and 4.66, respectively; all P > 0.05).