L-cysteine, poisoned by metal toxins, is critical to other biological responses, ones that also help block cancer development and make cancer cells die naturally. Apoptosis depends upon a series of enzyme activities called caspases. These enzymes are cysteine proteases, chewing on the cysteine bonds in their peptide and protein targets. Thus, without L-cysteine and L-cystine in the diet, and in a toxic environment precluding conversion to these amino acids from methionine, the protein upon which these caspases operate can't be made, so cancer cells don't have the infrastructure to die a natural death. Hence, the myth of the immortal cancer cell. These cells are simply too malnourished and/or toxic to die normally, so naturally they continue to grow..