Several intrinsic and extrinsic factors have been described to be implicated in the resistance to PDT, including those related with the treatment –location and production of PS-, cancer cells features –proliferation, adhesion and expression of certain genes implicated on the development of the tumour including p53 and components of shh/ptch and Wnt/β-catenin pathways18,31. The gene discussed is PTCH1; the disease is neoplasm.