Conversely, risk of CBC was elevated among women diagnosed with an ER‐negative first primary breast cancer who became postmenopausal during the at‐risk period (RR = 1.9, 95% CI: 1.02, 3.4; p for heterogeneity for ER‐positive vs. ER‐negative = 0.01). Here, ESR1 is linked to complete blood cell count.