Functions as a cation- and diacylglycerol (DAG)-regulated nucleotide exchange factor activating Ras through the exchange of bound GDP for GTP. May function in mast cells differentiation. Recruited to membranes upon activation by DAG. Expressed by mast cells and their progenitors (at protein level). Expressed at all stages in the development of mast cells. The phorbol-ester/DAG-type zinc finger mediates the binding and the functional activation by DAG. Due to an intron retention. Minor isoform which is not activated by DAG and probably not functional. Preferentially expressed in the C3H/HeJ strain. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the RASGRP family.