Accumulating evidence indicates that heregulins, EGF (epidermal growth factor)-like ligands, promote breast cancer cell proliferation and are involved in the progression of breast cancer towards an aggressive and invasive phenotype. However, there is limited information regarding the molecular mechanisms that mediate these effects. We have recently established that HRG (heregulin beta1) promotes breast cancer cell proliferation and migration via cross-talk with EGFR (EGF receptor) that involves the activation of the small GTPase Rac1. In the present paper we report that Rac1 is an essential player for mediating the induction of cyclin D1 and p21(Cip1) by HRG in breast cancer cells. Inhibition of Rac function by expressing either the Rac-GAP (GTPase-activating protein) beta2-chimaerin or the dominant-negative Rac mutant N17Rac1, or Rac1 depletion using RNAi (RNA interference), abolished the cyclin D1 and p21(Cip1) induction by HRG. Interestingly, the proliferative effect of HRG was impaired not only when the expression of Rac1 or cyclin D1 was inhibited, but also when cells were depleted of p21(Cip1) using RNAi. Inhibition of EGFR, PI3K (phosphoinositide 3-kinase; kinases required for Rac activation by HRG) or MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] also blocked the up-regulation of cyclin D1 and p21(Cip1) by HRG. In addition, we found that HRG activates NF-kappaB (nuclear factor kappaB) in a Rac1- and MEK-dependent fashion, and inhibition of NF-kappaB abrogates cyclin D1/p21(Cip1) induction and proliferation by HRG. Taken together, these findings establish a central role for Rac1 in the control of HRG-induced breast cancer cell-cycle progression and proliferation through up-regulating the expression of cyclin D1 and p21(Cip1).