MTAP and cancer: These enzymes are often downregulated in malignant tumors, resulting in a strong dependence of the cells on Met intake from the external environment.234,235 In addition, Met is broken down by the Met cycle, in which Met adenosine transferase 2A (MAT2A) is upregulated and highly active in tumors.236 MAT2A has been proposed as a lethal target in MTAP null tumors.