Circulating neutrophils are, by necessity, quiescent and relatively unresponsive to acute stimuli. In regions of inflammation, mediators can prime neutrophils to react to acute stimuli with stronger proinflammatory, pathogen-killing responses. In neutrophils G protein-coupled receptor (GPCR)-driven proinflammatory responses, such as reactive oxygen species (ROS) formation and accumulation of the key intracellular messenger phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ), are highly dependent on PI3K-, a Ras-GTP, and G coincidence detector. In unprimed cells, the major GPCR-triggered activator of Ras is the Ras guanine nucleotide exchange factor (GEF), Ras guanine nucleotide releasing protein 4 (RasGRP4). Although priming is known to increase GPCR-PIP 3 signaling, the mechanisms underlying this augmentation remain unclear. We used genetically modified mice to address the role of the 2 RasGEFs, RasGRP4 and son of sevenless (SOS)1/2, in neutrophil priming. We found that following GM-CSF/TNF priming, RasGRP4 had only a minor role in the enhanced responses. In contrast, SOS1/2 acquired a substantial role in ROS formation, PIP 3 accumulation, and ERK activation in primed cells. These results suggest that SOS1/2 signaling plays a key role in determining the responsiveness of neutrophils in regions of inflammation. safeguard, as only by prior exposure to a priming agent (chemoattractant, proinflammatory cytokine, or TLR agonist) neutrophils can maximally respond to a subsequent challenge with a ligand such as In recent years, neutrophils have been found infiltrating many types of tumors and it is becoming clearer that tumor-associated neutrophils (TANs) play a role in malignant disease. 3 Two types of TAN have been described with antagonistic affects (pro-tumor or anti-tumor), which are determined by factors expressed by the stromal cells or the tumor itself. Given the presence and important roles for TNFand GM-CSF in the tumor microenvironment and the host response to cancer, 4 it is very likely that the changes in neutrophil signaling and function induced by these ligands, and the process of priming, shape tumor progression.