The explained PPIX accumulation after 5-ALA supply occurs in healthy and cancer cells; nevertheless, some factors have been suggested to induce the differential PPIX accumulation observed between cancer and healthy cells [90]: (1) Availability of iron, (2) lower activity of ferrochelatase, (3) transferrin receptor expression, (4) altered expression of enzymes in heme biosynthesis, and (5) mitochondrial content. The gene discussed is FECH; the disease is cancer.