C/EBPδ transcriptional activator function is required for suppression of HC11 mammary epithelial cell colony growth. HC11 cells were transfected with expression constructs encoding C/EBPδ cDNA full length (FL), C/EBPδ bZIP domain deleted (DB) or C/EBPδ bZIP + DNA binding domain deleted (TA) constructs. HC11 cells transfected with the pcDNA3 vector alone yielded ~1,000 colonies (positive control). For all clonogenic assays, cells were cultured for 15 days in Complete Growth Media (CGM) plus G418. Surviving colonies were stained with a 0.5% crystal violet/20% methanol solution and counted. * = significantly different from HC11 cells transfected with the full length (FL) C/EBPδ expression vector at p < 0.05.