As ACE2, along with TMPRSS2 and exopeptidase CD26 (also known as DPP4), have critical roles in cell entry and viral infectivity, it has been proposed that global DNA methylation, including ACE2 gene methylation and histone modifications, may lead to different levels of susceptibility to viral infections, such as COVID-19 [148]. Here, DPP4 is linked to viral infectious disease.