The actions of estrogen (E2) are considered to be mediated through its nuclear E2 receptor (ER). In cancer development, growth factors are shown to act synergistically with E2. Recently, we found that the mitogen-activated protein kinase, activated by growth factors, phosphorylates human ERalpha and this phosphorylation potentiates the transactivation function of human ERalpha demonstrating a novel cross-talk between E2 and growth factor-signaling pathways. In this review, the molecular mechanism of this cross-talk is discussed.