Fig. 7. Ectopic activation of Ihh signaling in chondrocytes. (A) Transgene constructs used to drive ectopic expression of chick Ihh or Smo* using a UAS-Gal4 system. The UAS constructs contain the basic promoter region and the poly A signal from the murine Wnt1 gene, and the insulator sequence from the chick ß-globin gene. (B,C) In situ hybridization of indicated ³⁵S-labeled probes to tibial sections at E14.5 and E16.5 reveals ectopic Ihh signaling driven by chick Ihh (B) or Smo* transgene (C). Note that the chick Ihh probe reacts weakly with the endogenous murine Ihh in wild-type tibia. Expression of Smo* is detected by an alkaline phosphatase (AP) probe.