Cross-sectional studies have shown reduced gut microbiota diversity and compositional shifts from Bacteroides to Prevotella predominance after HIV-1 infection (Vazquez-Castellanos et al., 2015; Ling et al., 2016; Dillon et al., 2017; Lu et al., 2018), which have been linked to lower CD4 + T-cell count, higher inflammation, and increased immune activation (Vujkovic-Cvijin et al., 2013; Dinh et al., 2015). The gene discussed is CD4; the disease is HIV-1 infection.