A tetracycline regulatory unit and polyadenylation signals were inserted upstream of a tetO and minimal CMV promoter. This was followed by a floxed PGK-neomycin-polyA cassette and then by a cDNA for the human smoothened homolog containing a Trp to Leu mutation in codon 535 and fused to YFP. Expression of the SMO* knock-in occurs only after cre excision of the PGK-neo cassette which can be accomplished by tamoxifen treatment to activate cre expression from the TG(CAG-cre/Esr1*)5Amc transgene carried by the YFP3-1 ES cell line used to create this mutation. (J:165962)