Estradiol rapidly activates Src as well as the Src-dependent pathway in human mammary cancer-derived MCF-7 cells, in human prostate cancer-derived LNCaP cells and in Cos cells transiently expressing hERs [EMBO J. 15 (1996) 1292; EMBO J. 17 (1998) 2008]. In addition, estradiol immediately stimulates, yes, an ubiquitous member of the Src kinase family, in human colon carcinoma-derived Caco-2 cells [Cancer Res. 56 (1996) 4516]. Progestins and androgens activate the same pathway in human mammary and prostate cancer-derived cells [EMBO J. 17 (1998) 2008; EMBO J. 19 (2000) 5406]. We observed that estradiol also stimulates the phosphatidylinositol-3-kinase (PI3K)/AKT pathway in MCF-7 cells [EMBO J. 20 (2001) 6050]. In these cells, activation of the Src- and the PI3 K-dependent pathways is simultaneous and mediated by direct interactions of the two kinases with ERalpha. The signalling pathway activation by sex-steroid hormones leads to DNA synthesis and cell growth in human mammary and prostate cancer-derived cells [EMBO J. 19 (2000) 5406; EMBO J. 20 (2001) 6050; EMBO J. 18 (1999) 2500]. Furthermore, androgen stimulation of NIH3T3 fibroblasts activates the same pathways triggered by this hormone in LNCaP cells and promotes the S-phase entry or cytoskeleton changes in these cells [J. Cell Biol. 161 (2003) 547]. All the described effects are rapid and require classic steroid receptors, but, surprisingly, not their transcriptional activity. Indeed, a transcriptionally inactive mutant of hER mediates the estrogen-stimulated DNA synthesis of NIH3T3 fibroblasts [EMBO J. 18 (1999) 2500]. Furthermore, AR in NIH3T3 cells does not enter nuclei and is unable to respond to the hormone with transcription stimulation, whereas it activates signaling pathways and triggers important biological responses. Signaling pathway activation by steroids has also been described by other groups under different experimental conditions and/or in different cell types. In these cells, steroid stimulation triggers various effects, such as neuroprotection, vasorelaxation or bone protection [J. Neurosci. Res. 60 (2000) 321; Nature 407 (2000) 538; J. Cell Biochem. 76 (1999) 206]. Analysis of the mechanisms responsible for the hormone-dependent and steroid receptor-mediated pathway activation in epithelial as well as stromal cells reveals immediate association of steroid receptors with extranuclear signaling effectors [EMBO J. 17 (1998) 2008; Cancer Res. 56 (1996) 4516; EMBO J. 19 (2000) 5406; EMBO J. 20 (2001) 6050; J. Cell Biol. 161 (2003) 547]. These results further highlight the central role of the hormone-regulated protein-protein interactions in the steroid action. They also offer the possibility of interfering with important activities of hormones, such as proliferation or survival, cytoskeleton changes as well as invasiveness and vasorelaxation, without affecting the steroid effects that depend on receptor transcriptional activity.