Anarchic cell proliferation, observed in some leukemia and in breast and ovarian cancers, has been related to dysfunctioning of cytoplasmic or receptor tyrosine kinase activities coupled to p21 Ras. The growth factor receptor-bound protein 2 (Grb2) adaptor when complexed with Sos (Son of sevenless), the exchange factor of Ras, conveys the signal induced by tyrosine kinase-activated receptor to Ras by recruiting Sos to the membrane, allowing activation of Ras. This review shows how it is possible to stop the Ras-deregulated signaling pathway to obtain potential antitumor agents. Grb2 protein is comprised of one SH2 surrounded by two SH3 domains and interacts by means of its Src homology (SH2) domain with phosphotyrosine residues of target proteins such as the epidermal growth factor (EGF) receptor or the Shc adaptor. By means of its SH3 domains, Grb2 recognizes proline-rich sequences of Sos, leading to Ras activation. Inhibitors of SH2 and SH3 domains were designed with the aim of interrupting Grb2 recognition. On the one hand, using structural data and molecular modeling, peptide dimers or "peptidimers", made up of two proline-rich sequences from Sos linked by an optimized spacer, were developed. On the other, using the structure of the Grb2 SH2 domain complexed with a phosphotyrosine (pTyr)-containing peptide and molecular modeling studies, a series of N-protected tripeptides containing two phosphotyrosine or mimetic residues, with one pTyr sterically constrained, were devised. These compounds show very high affinities for Grb2 in vitro. They have been targeted into cells showing selective antiproliferative activity on tumor cells. These results suggest that inhibiting SH2 or SH3 domains of signaling proteins might provide antitumor agents.