An early in vitro study showed that breast cancer cells (MCF7) grown under conditions of chronic glucose deprivation (0–1 mM glucose) produced higher levels of PpIX than those grown under standard conditions.54 Co-incubating glycolysis inhibitors with 5-ALA significantly reduced intracellular levels of PpIX in breast cancer cells (MCF7).55 Inhibiting glycolysis depletes ATP levels and has been shown to inactivate ABC transporters,56 so it is possible that a reduced flux of precursors led to this reduction in PpIX. Here, ABCG2 is linked to breast cancer.