CRISPR (clustered regularly interspaced short palindromic repeat) genome editing technology identified CCR9 and SLC6A20 as plausible causal genes associated with COVID-19 severity [26], while joint genome-scale CRISPR loss-of-function screens and expression quantitative trait locus analysis pointed to SLC6A20 and CXCR6 as target genes [27]. Here, CCR9 is linked to COVID-19.