Phosphorylation of SAMHD1 negatively regulates its HIV-1 restriction function. SAMHD1 only exhibits antiretroviral activity when expressed in non-cycling cells because SAMHD1 is unphosphorylated in non-cycling cells. Phosphorylated threonine at residue 529 (T529) of human SAMHD1 is repressive for HIV-1 restriction in cycling cells. CDK1: cyclin-dependent kinase 1; CycA2: Cyclin A2. The letter P in the diagram indicates phosphorylation of T592 of human SAMHD1. PMA: phorbol 12-myristate 13-acetate.