Guanine nucleotide exchange factor (GEF) for RAP1A, RAP2A and MRAS/M-Ras-GTP. Its association with MRAS inhibits Rap1 activation (By similarity). In the embryo, expressed in young neurons of the developing telencephalon, diencephalon and hindbrain. Not expressed in progenitor cells in the ventricular zone. Expression is first detected at embryonic day 13. Down-regulated in nerve growth factor-treated PC12 cells.