Stable over-expression of several Hh components drives constitutive pathway activity. WT iMEFs were infected with retrovirus encoding, hShh-GFP, hGli1-GFP, ΔNmGli2-GFP, hSmo-GFP or an empty GFP IRIS vector. Stable over-expresser cell lines produced by GFP sorting were plated at confluence. Following 24 hrs, expression of Ptc1 was determined by Real-Time RT-PCR. Values represent the mean ± SEM of three replicate experiments, * indicates P ≤ 0.05 (paired t-test) vs. GFP iMEFs.