AK2 and neoplasm: Indeed, the level of AK2 in MYC-high breast tumours was highly significantly elevated, as were other genes in the nucleotide biosynthesis pathways (Fig. 7F), indicating that the AK2 gene exhibits positive selection in tumours expressing high levels of MYC. Thus, TCA cycle activity and mitochondrial energetics are critical determinants of the apoptosis triggered by glutamine withdrawal.