The mechanism of action that leads to the selective accumulation of 5-ALA in GBMs and high grade gliomas cells depends on the following three factors: (1) the disruption of the blood–brain barrier exhibited by cancer cells, which allows 5-ALA to penetrate brain tissue, (2) the property of cancer cells to have increased metabolism of porphyrins and formation of PpIX due to the hyperactivity of 5-ALA synthase, and (3) the dysfunction of ferrochelatase (FECH) in tumor cells, an enzyme that converts PpIX into heme in normal brain cells [12]. The gene discussed is FECH; the disease is neoplasm.