They are modulated by estrogen, and can contribute to BC control by endocrine therapy in preclinical models.<h4>Methods</h4>To elucidate whether changes in monocyte subsets are associated with treatment and response, we investigated peripheral blood samples of 73 postmenopausal women with estrogen receptor (ER) positive BC, who received aromatase inhibitor therapy with or without the mucin-1 vaccine tecemotide in the ABCSG34 trial. Here, MUC1 is linked to breast cancer.