Cyclin T1 also known as CCNT1 is a member of the highly conserved cyclin C subfamily. Cyclin T1 plays a crucial role in regulating transcription elongation by forming a complex with cyclin-dependent kinase 9 (Cdk9), which is essential for phosphorylating the C-terminal domain of RNA polymerase II. This phosphorylation enables RNA polymerase II to transition from transcription initiation to elongation, facilitating mRNA production. Cyclin T1 also interacts with HIV-1 Tat protein, enhancing its binding affinity to the viral TAR RNA structure, which is critical for recruiting the cyclin T1/Cdk9 complex to RNA polymerase II. The specificity of cyclin T1 in this interaction highlights its importance in both cellular transcription processes and viral pathogenesis, making cyclin T1 a significant target for research in gene expression and HIV biology.