As expected, FceRI activation induced robust changes in gene AZ191 expression in all four types of mast cells studied. At the given cut-off level, 209 genes showed different expression in nonactivated NTAL KO cells. It is remarkable that no differences in gene expression were noticed between Agactivated WT and WT pLKO when similar criteria for analysis of differential gene expression were used. This confirms that infection and puromycin selection had no significant effect on the data obtained from lentivirally infected and activated cells. This is in marked contrast with comparison of RNA from activated cells with NTAL KO vs WT and NTAL KD vs WT pLKO, where 194 and 165 genes, respectively, were found differentially expressed. When comparing expression levels in various cell types we noticed that the degree of overlap between nonactivated and activated NTAL KO and KD cells was rather modest. This could be due to methodological differences in production of NTALdeficient cells. However, it should be kept in mind that although lentiviral infection itself and puromycin selection caused differential expression of some genes, as can be deduced from the observed differences in gene expression between nonactivated WT and WT pLKO cells, this difference disappeared in activated cells. Thus, lentiviral infection and puromycin selection did not contribute to the differences observed, at least in activated cells. A hypothetical simplified model on the role of NTAL in mast cell activation and transcriptional regulation in WT and NTALdeficient cells is shown in Figure 9. In nonactivated WT cells both adaptor proteins, NTAL and LAT, as well as FceRI b and c subunits are only weakly tyrosine phosphorylated, because of the equilibrium between kinases and phosphatases and/or Hydrocortisone acetate decreased access of the kinase to their substrates. Quiescent cells also exhibit low i and standard gene expression. After Ag-mediated activation there is enhanced tyrosine phosphorylation of FceRI b and c subunits by LYN and SYK kinase. Activated SYK phosphorylates NTAL and LAT and this leads to further propagation of the activation signal, increased i and dramatic changes in gene expression by so far not fully understood mechanism.