CTSB and neoplasm: In the oxidizing extracellular environment, free cysteines combine to form cystine (270), and, thus, releasing cathepsin B into the extracellular matrix would create an abundance of extracellular cystine, which we theorize the tumor cells then uptake and reduce back to cysteine to produce pheomelanin to provide increased energy to the tumor cells, which the tumor cells then use to survive, proliferate, and metastasize.