Fig. 1. Existing rhombic lip precursors fail to form an EGL in Math1^{ß-gal/ß-gal} cerebellum. Whole-mount X-Gal staining of brains from E14.5 (A-D) and E16.5 (E,F) mice. Expression of a lacZ reporter under the endogenous control of Math1 promoter is seen in the rhombic lip of E14.5 Math1^ß-gal/+ (Het, A,C) and Math1^{ß-gal/ß-gal} (Null, B,D). Stained progenitors are seen in both genotypes, although the rhombic lip seems smaller in Math1 null cerebellum. At E16.5, lacZ expression is detected in CGC progenitors migrating over the cerebellar surface to generate the EGL in Math1^ß-gal/+ (E) but not in a Math1^{ß-gal/ß-gal} littermate (F). The rhombic lip was dissected out from E14.5 Math1^ß-gal/+ brain and subjected to X-Gal staining (G). The large proportion of stained cells indicates that the isolated tissue is enriched with CGC progenitors. RT-PCR on the isolated rhombic lip verifies the expression of lacZ, Math1 and Zipro1 (H). + and - indicate the presence and absence of reverse transcriptase, respectively. (A,B,E,F) Dorsal views; (C,D) Lateral views. rl, rhombic lip; IV, fourth ventricle of the brain, EGL, external granule layer. Scale bars: 1 mm.