GEF
GEF, also known as Sos, is a protein linked with the RAS pathway. It opens up the nucleotide-binding pocket of Ras. This releases GDP and allows GTP to attach to Ras, thus activating it.
Berg J, Stryer L, Tymoczko J. (2011) Biochemistry, 7th Edition. International Edition, Basingstoke: Palgrave Macmillan. Page 431.