CXCR4 and Immunodeficiency: Compared to the other two participants who began ART shortly after HIV-1 diagnosis, participant 2 was diagnosed with HIV-1 25 years prior to starting antiretroviral therapy and died 10 months after ART commencement; therefore, viral CXCR4 tropism most likely resulted from ‘coreceptor switch’ frequently occurring during advanced stages of immune deficiency (Connor et al., 1997).