Apart from LDHA/B, elevated levels of the lactate transporter MCT1/4 have also been observed in lung cancer cells with LKB1 inactivation (96, 97), suggesting that intracellular lactate is not only incorporated into the TCA cycle but that extracellular lactate released by neighboring cancer cells can be taken up and incorporated into the TCA cycle as an energy source (Figure 3A). This evidence concerns the gene STK11 and cancer.