MTAP and hepatocellular carcinoma: By contrast, expression of methylthioadenosine phosphorylase (MTAP), which catalyzes the conversion of 5′-deoxy-5′methylthioadenosine (MTA) to adenine and methylthioribose 1-phosphate, is reduced in many tumors including malignant melanoma (19) and hepatocellular carcinoma (20) due to either gene disruption by chromosomal rearrangement or epigenetic silencing.