Fig. 2. En1 is required prior to E9.0 to pattern the anterior cerebellum and after E9.0 to form the posterior midbrain. (A) Schematic illustrating the expression profile of En alleles in the mes/r1 region of En1^flox/Cre mice. Note that only one functional En1 allele is present because the En1-Cre allele is an En1-null. (B,C) Dorsal view of posterior adult brains of (B) En1^Cre/+ and (C) En1^flox/Cre mice. (D-I) Cresyl Violet-stained sagittal sections of the brains were taken at the level of (D-F) the vermis and (G-I) hemispheres as indicated in B,C by the red and black dashed lines, respectively. E,H and F,I are sections taken from brains in B and C, respectively. Arrow in F indicates the predominant phenotypes of fusion of vermis folia I-II and III and the red asterisk indicates a slight truncation of the inferior colliculus. In some En1^flox/Cre mice, a normally foliated Cb was also seen (D,G). H, hemisphere; V, vermis; Pv, paravermis; Cb, cerebellum; IC and SC are the inferior and superior colliculus, respectively outlined by red and green dashed lines. Vermis and hemisphere folia are indicated by roman numerals (I-X) and letters (A, S, CI, CII, Pm, P), respectively.