Estrogen receptors (ERα and ERβ) are estrogen-regulated transcription factors that play important roles in the development and progression of breast cancer. The biological function of ERs has been shown to be modulated by ER-interacting proteins. However, the ER-interacting proteins that not only activate MAPK and AKT, two important growth regulatory protein kinases, but also increase growth related estrogen-responsive gene expression remain unknown. Here, we report that hematopoietic PBX-interacting protein (HPIP) interacts both with ERα and with ERβ, and increases ERα target gene expression through activation of MAPK and AKT and enhanced ERα phosphorylation. ERβ inhibits ERα target gene expression, possibly by competition of ERβ with ERα for binding to HPIP, and by a decrease in available ERα for HPIP binding through the interaction of ERβ with ERα. Furthermore, HPIP increases breast cancer cell growth. These data suggest that HPIP may be an important regulator in ER signaling and that the relative ratio of ERβ to ERα may be important for HPIP function.