SMARCA1 and breast cancer: an Using an epigenome CRISPR knockout screen, seminal work by Xu, et al.[101] has identified loss of ARID1A, a member of the SWI/SNF chromatin remodeling family, as a major driver of clinical resistance to the ER antagonist and selective ER degrader fulvestrant in breast tumors.[101] Mechanistically, loss of ARID1A, was shown to cause major epigenetic remodeling of transcriptional networks during breast cancer progression, leading to a fundamental switch in cell state, from ER‐dependent (and fulvestrant‐responsive) luminal cells to an ER‐independent basal‐like breast cancer cell type.