other reason for the different CX-4945 site cerebellar phenotypic outcomes of our Fgfr2 cKO and the previously generated conditional Fgfr2 mutant mice might be the different gene targeting strategies used for generating these mice, although they should all result in the absence of a functional FGFR2 receptor in the developing CbA. FGF9/FGFR2-mediated signaling might act as a positioning cue for migrating BG cells BG cells were located ectopically in the anterior EGL/ML of the Fgfr2 cKO cerebella, indicating that FGFR2 signaling is necessary for their proper positioning within the PCL. The radial migration of BG precursors and cells from the VZ toward the PCL starts at,E14 and reaches a peak between E1516 in the mouse, the time interval when Fgfr2 transcription initiates in the developing CbA. SHH secreted from PCs is a potent chemoattractant for BG cells that strongly promotes their migration. The normal transcription of Shh in PCs of the mutant CbA suggests that this guidance cue is not affected in the Fgfr2 cKO embryos. The migration of BG cells, however, must be inhibited once these cells have reached their final destination in the PCL to prevent their ectopic positioning beyond this layer. We therefore hypothesized that an FGF signal emitted from the EGL and/or PCL might provide such a stop signalto migrating BG cells. One potential candidate was FGF9 expressed in GCPs and PCs and required for the proper positioning of BG cells in the PCL, although other FGFs expressed within the EGL or PCL might have a similar function. The outward migration of RG/BG precursors/cells from CbA microexplants was indeed inhibited after FGF9 treatment of these explants, whereas FGFR blockade promoted the outward migration of RG/BG precursors/cells for longer distances from the explants. These results strongly suggest that RG/BG precursors/ cells fail to detect the probably concentration-dependent FGF9 stop signalfrom the EGL/PCs in the absence of FGFR2mediated signaling, and therefore migrate beyond their normal position within the PCL. Altogether, our findings thus reveal the specific pro-differentiation, anti-apoptotic and cell positioning functions of FGFR2-mediated signaling in RG/BG precursors/ cells during cerebellar development in the mouse, and might provide new mechanistic insights to the pathogenesis of cerebellar ataxias. Supporting Information Incomplete penetrance and Fgfr1 as a genetic modifier of the Fgfr2 cKO cerebellar phenotype The cerebellar defects of the Fgfr2 cKO mice are not completely penetrant and may have been missed inadvertently in previous FGFR2 in Bergmann Glia Development Pax62 RG/BG precursors/cells among the total number of migrating Ccnd1+ and/or Pax6+ cells in each 50-mm bin in control-, FGF9- or SU5402-treated microexplant cultures. Values represent the average proportion of Ccnd1+/ Pax62 RG/BG precursors/cells among the total number of migrating Ccnd1+ and/or Pax6+ cells in each 50-mm bin and for each treatment, and the 95% confidence interval estimated with a logistic model. Purkinje cell layer; rH, rostral hindbrain; rl, rhombic lip; Tg, 
tegmentum; VZ, cerebellar ventricular zone. Scale bar: 200 mm. Acknowledgments We thank M. Sendtner for the 23742272 Fgfr2lox/lox mice, and A. Folchert, M. Homburg, S. Laab and B. Sperling for expert technical assistance. The monoclonal anti-Pax6 antibody was obtained through the 17628524 Developmental Studies Hybridoma Bank under the auspices of the National Institute of Child Health and Human Development and
