GREB1 (growth regulation by estrogen in breast cancer 1) is an estrogen-responsive gene that contains three EREs located 1.6, 9.5 and 21.2 kb upstream of the transcriptional start site (Ghosh et al 2000; Lin et al, 2004; Rae et al, 2005; Deschenes et al, 2007; Sun et al, 2007). By ChIP, all three EREs are bound by ESR1 and the transcriptional co-activator NCOA3 (also known as SRC3). Although this binding occurs even in the absence of estradiol treatment, binding is enhanced after estrogen stimulation (Sun et al, 2007). Estrogen-dependent GREB1 expression also depends on removal of the repressive H3K9 methlyation mark by KDM4B (Kawazu et al, 2011; Gaughan et al, 2013). Estrogen stimulation increases the occupancy of RNA polymerase II at the GREB1 gene and may promote transcription through the formation of chromatin loops. Estrogen stimulation also increases the level of H4 acetylation at the promoter (Sun et al, 2007; Deschenes et al, 2007). In addition to ESR1 and NCOA3, Kruppel-like finger (KLF) protein ZNF217 has also been shown to bind to ESR1 and enhance recruitment to GREB1 EREs. ZNF217 overexpression is associated with anchorage independent growth in MCF7 cell lines (Nguyen et al, 2014). Although both NCOA3 and ZNF217 have been shown to interact with the GREB1 EREs, no study has examined co-occupancy of the GREB1 enhancer by these two regulators.