A study in South Africa, assessing the burden of AHD over 10 years, found that ART-naïve patients initially constituted the majority of those with CD4 count < 50 cells/mm3; but a shift occurred over the decade, and the contribution of ART experienced patients to this group increased from 14.3% to 56.7%, becoming the largest proportion of those with AHD. The gene discussed is CD4; the disease is mitochondrial DNA depletion syndrome 4a.