ESR1 and breast cancer: FOXA1 is a ‘pioneer' forkhead transcription factor that can directly bind condensed chromatin, displace repressive linker histones and recruit other transcription factors to promote transcription.7, 8 It is required by ERα as a cofactor for chromatin binding9 and has been reported to regulate nearly 50% of all oestrogen receptor (ER)-target genes.10 Thus, FOXA1 and ERα constitute a major proliferative and survival axis for breast cancers, specifically of the luminal A subtype.