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). Specifically expressed in mononuclear leukocytes. Highly expressed in myeloid cells compared to lymphoid cells. Also detected in heart, skeletal muscle, spleen, liver, placenta and lung. Not detected in brain. Isoform 1 is the major isoform in normal individuals. Isoform 2 is more significantly expressed in a patient with asthma. Isoform 3 is more significantly expressed in a patient with asthma and a mastocytosis patient. Expressed early during development. Detected in all fetal tissues tested. The phorbol-ester/DAG-type zinc finger mediates the binding and the functional activation by DAG. Probably inactive. Probably inactive. Belongs to the RASGRP family.