One proposed mechanism is that Cathepsin B expression in melanoma stimulates fibroblast activation through a transforming growth factor β (TGFβ)-dependent pathway, and this stromal involvement increases melanoma traversal through the basement membrane; however, this acts only as an additive mechanism, as Cathepsin B also supports melanoma invasiveness without the presence of fibroblasts [44]. The gene discussed is CTSB; the disease is melanoma.