The genome-wide association meta-analyses that consisted of 49,562 COVID-19 patients from 46 studies across 19 countries indicated that specific mutations of the tyrosine kinase 2 (TYK2) gene could enhance the risk of becoming critically ill with COVID-19, while individuals who carried a variant in the dipeptidyl peptidase 9 (DPP9) gene were at elevated risk of developing critical disease (191, 192). Here, DPP9 is linked to COVID-19.