C3G/GRF2 is a 130-140 kDa guanine-nucleotide releasing protein for Ras proteins, Rap1 and R-Ras. It belongs to the adaptor-type Src homology (SH)2-containing molecules. C3G was originally identified by virtue of its binding to the SH3 domain of the adapter protein Crk. Binding to Crk is mediated by a centrally located proline-rich region. C3G has also been reported to bind directly to Cas. Phosphorylation of C3G on tyrosine is reported to increase the nucleotide exchange activity for Rap1