CCNT1, also termed CycT1, is a host factor essential for HIV-1 replication in CD4 T cells and macrophages. It is a regulatory subunit of the cyclin-dependent kinase pair (CDK9/cyclin-T1) complex, also called positive transcription elongation factor B (P-TEFb), which is proposed to facilitate the transition from abortive to productive elongation by phosphorylating the CTD (C-terminal domain) of the large subunit of RNA polymerase II (RNA Pol II). CCNT1 contains two cyclin boxes. This model corresponds to the second one. The cyclin box is a protein binding domain.