(A) Model of normal mammalian telencephalic development. On the left side, the PrCP is represented by a white rectangle. The blue square around it highlights those steps known to be critical in the pathogenesis of alobar HPE. Toward the right side of the diagram, genes known to be important during subsequent steps of forebrain development are indicated. The orange rectangle highlights steps and genes important for semilobar HPE, the green rectangle highlights those important for MIH, and the gray rectangle highlights those important for microforms of HPE. Solid lines represent those processes that have been demonstrated and dashed lines represent those processes that have not yet been directly proved. To better understand the regional relationships between some of those critical genes, their normal expression patterns in the telencephalon at E9.0 and E10.5 are illustrated in B and C, respectively. C is adapted with permission from Journal of Neuropathology and Experimental Neurology (106). Hnf3b, hepatocyte nuclear factor 3β; Otx2, orthodenticle homolog 2; Pax6, paired box gene 6; Wnt8b, wingless-related MMTV integration site 8b.