The ETS transcription factor ELF5 drives mammary alveolar development in preparation for lactation by forcing differentiation within the progenitor cell population. In luminal A breast cancer, early disease progression is predicted by high levels of ELF5, and in preclinical models elevated ELF5 is associated with its two key features, the acquisition of resistance to endocrine therapy and increased metastasis. Here we demonstrate using chromatin immunoprecipitation sequencing that ELF5 binding overlaps with FOXA1 and ER at enhancers and promoters, and when elevated causes FOXA1 and ER to bind to new regions of the genome involved in resistance to endocrine therapy.