SAMHD1 provides a potent post-entry block against immunodeficiency viruses in non-cycling cells. Its dNTP-triphosphohydrolase activity lowers the cellular dNTP pool, preventing viral reverse transcription. HIV-2/SIVs use their Vpx and Vpr accessory proteins to modify the host cell’s CUL4A–DDB1–DCAF1 ubiquitin ligase specificity towards SAMHD1, resulting in its proteasomal degradation and ultimately raising dNTP levels, making the cells permissive to viral replication. Sequence similarity and comparative functional analysis suggest that the ancestral HIV-1 accessory protein Vpr uses a similar mechanism to exploit the CUL4A–DDB1–DCAF1 system to induce proteasomal degradation of an as yet undiscovered cellular factor whose absence causes cell cycle arrest in the G2 phase, promoting viral replication and pathogenesis in vivo.