ZIC4 and Hydrocephalus: The cerebral cortex of Zic4Cre;Ext1Fl/Fl brains was thinned and the ventricles were enlarged (cf. Fig. 4A,C) and this hydrocephalus-like phenotype is intriguing because the cerebral cortex is not of the Zic4 lineage, indicating a non-cell-autonomous mechanism by which HS regulates cerebral cortex development.