Fig. 3. Homology region-c (HR-c) is responsive to Shh signaling. Neural explants isolated from wild-type (A,C) or Shh^-/- (B,D) embryos carrying Rc9 were cultured in the presence (+) or absence (–) of N-Shh for 30 hours. A few wild-type cells showed X-gal staining when cultured in the absence of N-Shh (possibly reflecting their prior exposure to Shh in vivo), however this number was significantly increased when cultured in the presence of N-Shh (compare A with C, n=16). No cells showed X-gal staining in explants derived from Shh^-/-, RC9⁺ embryos grown in the absence of N-Shh (B), in marked contrast to the significant number of positive stained cells from the corresponding explant grown in the presence of N-Shh (D, n=3).