The study identifies SAMHD1 as the restriction factor that makes human dendritic and myeloid cells largely resistant to HIV-1 infection. Vpx, an auxiliary protein of HIV-1 and SIV, counteracts this restriction by inducing the proteasomal degradation of SAMHD1. The authors show that silencing SAMHD1 in non-permissive cell lines increases HIV-1 infection, while overexpression of SAMHD1 in sensitive cells inhibits it. The phosphohydrolase activity of SAMHD1 is crucial for its antiretroviral function. Vpx-mediated relief of SAMHD1 restriction is abolished in SAMHD1-negative cells, and silencing SAMHD1 in monocytic-derived dendritic cells enhances their susceptibility to HIV-1 infection. These findings highlight SAMHD1's role in HIV-1 infection and suggest that modulating its function could impact the immune response to HIV-1.The study identifies SAMHD1 as the restriction factor that makes human dendritic and myeloid cells largely resistant to HIV-1 infection. Vpx, an auxiliary protein of HIV-1 and SIV, counteracts this restriction by inducing the proteasomal degradation of SAMHD1. The authors show that silencing SAMHD1 in non-permissive cell lines increases HIV-1 infection, while overexpression of SAMHD1 in sensitive cells inhibits it. The phosphohydrolase activity of SAMHD1 is crucial for its antiretroviral function. Vpx-mediated relief of SAMHD1 restriction is abolished in SAMHD1-negative cells, and silencing SAMHD1 in monocytic-derived dendritic cells enhances their susceptibility to HIV-1 infection. These findings highlight SAMHD1's role in HIV-1 infection and suggest that modulating its function could impact the immune response to HIV-1.