To better understand how disrupted HH/Gli signaling results in tracheomalacia, we analyzed a series of conditional mouse mutants in which we either deleted the HH receptor Smo, which effectively removes GliA, or we ectopically expressed Gli3TFlag/+, which, like PHS patients, has elevated Gli3R activity but preserved GliA function (Vokes et al., 2008). The gene discussed is SMO; the disease is Pallister-Hall syndrome.