ARID1A and breast cancer: Loss of either ARID1A or BRG1 SWI/SNF subunits is frequently observed in diseases in which PGR expression is lost, including breast cancer, in which ARID1A is mutated in 5% of cases [88] and ARID1A and BRG1 are known to play a role in response to ESR1 antagonists [89].