Previously we and others showed that bone marrow-derived mast cells from Ntal-/- mice were hyper-responsive to FceRI stimulation, whereas BMMCs from Lat-/- mice were hypo-responsive. Interestingly, loss of both NTAL and LAT caused stronger inhibitory effect on FceRI-mediated degranulation than loss of LAT alone. This suggested that NTAL could also have a positive regulatory role in FceRI signaling, manifested only in the absence of LAT. In contrast to studies with cells from mice with NTAL knock out, NTAL knockdown by RNAi in human mast cells and also in rat basophilic leukemia cells resulted in impaired degranulation; it implies that NTAL has positive regulatory roles in these cells even in the presence of LAT. To rigorously examine the regulatory role of NTAL in murine mast cells signaling and to test the contribution of compensatory developmental alterations in mast cells from NTAL KO mice, we prepared BMMCs with NTAL KO or KD and the corresponding controls and cultured them under comparable welldefined conditions. For functional comparison of mast cells with NTAL KO or KD we examined several parameters characteristic for FceRI signaling including degranulation, calcium mobilization, tyrosine phosphorylation of LAT and ERK, depolymerization of Platycodin-D filamentous actin, and chemotaxis. The results obtained with the NTAL KD BMMCs were very similar to those of NTAL KO cells and thus support the notion that in murine mast cells NTAL is predominantly a negative regulator of FceRI signaling and that compensatory developmental alteration do not contribute to this phenotype. To gain a better understanding of the genes that are regulated through NTAL-dependent pathways, we further examined the gene Hexyl Chloroformate expression profiles of resting and Ag-activated BMMCs with NTAL KO or KD and corresponding controls. Several genes have been identified that differ by a factor of 1.8 and higher in their expression in resting and FceRI-activated NTAL-deficient cells when compared to wild type cells. Through gene ontology analysis we identified a subset of NTAL-dependent genes, which were related to metabolism and biosynthetic processes.