ARID1A and breast carcinoma: Several repressive factors and complexes were linked with ER signaling and we will describe 3 such examples (AT rich interactive domain 1A [ARID1A-BAF], NuRD and Polycomb group), selected based on frequency of alterations in ER+ breast cancer (ARID1A); a functional link with ER biology and tamoxifen response (NuRD); and an unexpected role for a transcriptional repressor in the activation of ER-bound enhancers (Polycomb).