HEXIM1 and HIV-1 infection: CDK9/cyclin T1 associates with 7SK RNA [33], [34] and a hexamethylene bisacetamide (HEXIM1) protein [35], [36] and this interaction inhibits the activity of CDK9 [33], [34], [35], [36] Recently, PP1α and protein phosphatase 2B (PP2B) were shown to disrupt the interaction between CDK9/cyclin T1 and 7SK RNA/HEXIM1, thereby inducing the activity of CDK9/cyclin T1 [12] Currently, the regulatory subunit that targets PP1 to CDK9 remains unknown, although in HIV-1 infection, the viral Tat protein apparently serves this role.