nce that the smaller,82 kDa BRaf protein we detect in our cKO mice is a nonfunctional kinase. The most prominent function of neural progenitors in the subgranular zone of the dentate gyrus in the hippocampus of adult mammals, including humans, is the generation of new granule cell neurons. It is a striking example of structural anatomical reorganization of a neuronal circuit leading to functional changes at the level of synapse formation. Notably, neurogenesis in the SGZ of the dentate gyrus is not a simple replacement event to compensate for the loss of cells, but instead leads to the addition of new glutamatergic neurons in the granule cell layer. The functional importance of adult neurogenesis has been demonstrated in mice where impaired spatial memory function was noted after elimination of the stem cells. A small fraction of Sox2-positive neural progenitors in the subgranular zone has the potential to differentiate into astrocytes. Using BrdU birth-dating in conjunction with GFAP immonostaining, we have observed an increased fraction of radial glia precursor cells. The fraction of horizontal BrdU/GFAPpositive cells corresponding to Lck Inhibitor custom synthesis differentiated astrocytes was unchanged in the absence of BRaf indicating that the cell fate of Ablation of BRaf Impairs Neuronal Differentiation 9 Ablation of BRaf Impairs Neuronal Differentiation 10 Ablation of BRaf Impairs Neuronal Differentiation the progenitor cell progeny did not change from neuron to glia, as has been observed for example upon transient Notch activation. The use of Cre 
transgenic mice that are specific for neural stem cells and can be activated in adult animals by the hormone tamoxifen, such as CreGli1-CreERT2 transgenic mice may provide a means for a more detailed study of the role of BRaf in adult hippocampal neurogenesis. The increased number of BrdU positive cells in the 11821021 subgranular zone of P20 cKO male compound 9128839 mice could reflect a reactive response. Our immunostaining data indicate that BRaf is expressed in the SGZ band of the dentate gyrus, albeit at lower levels than in the differentiated granule neurons. It has been observed before that the mRNA encoding BRaf is expressed in the dentate gyrus and upregulated after longterm potentiation. The increased fraction of BrdU positive cells that have lost Ki-67 expression is intriguing. The Ki-67 antigen is a nuclear and nucleolar protein expressed during all active phases of the cell cycle and thus is a marker for proliferating precursor cells, but is absent from differentiated neurons or quiescent stem cells. In our Western blot experiments of total embryonic, as well as postnatal brain extracts we observed two BRaf bands of,92 and,89 kDa, respectively. Two BRaf bands have been noted before. We further observed that these two bands are absent in tissue extracts from embryos with homozygous exon 3 deletion. These genetic data do not support the idea that the known splicing in exons 8b/9b is the basis for the isoforms observed in the immune blots but rather argue in favour of a splicing within exon 3 of the BRaf gene. Sequencing of RTPCR products confirmed the existence of an intron in exon 3 that was spliced from the mRNA such that in the encoded smaller BRaf protein 33 amino acid residues would be missing. This intron/spliced-out segment starts with GT at the 59-splice site and ends with an AG at the 39 splice site and conforms to the canonical splicing pathway. The encoded protein domain is situated N-terminal in close proximit
