CD4 and HIV infectious disease: This model is supported by our recent reports that deficiency of ATM (an important DNA damage and repair enzyme) during HCV or HIV infection promotes DNA damage, CD4 T cell dysfunction and apoptosis, likely due to the unrepaired DNA damage (Zhao et al., 2018; Zhao et al., 2019; Khanal et al., 2020; Cao et al., 2022).