This concept is supported by a number of experimental data showing that: (i) tumour cells, but not the surrounding stroma, express HH ligands and downstream HH signalling components (e.g. PTCH1, GLI1) (e.g. Refs 67, 74, 78); (ii) tumour cell growth could be inhibited by RNAi-mediated knockdown of SMO or GLI1 and by treatment with cyclopamine (a SMO antagonist) in vitro and in xenograft models in vivo; (iii) metastatic growth could be prevented in vivo upon RNAi-mediated knockdown of SMO or GLI1 (Ref. Here, PTCH1 is linked to neoplasm.