Active and inactive P-TEFb complexes. A) Active P-TEFb complexes. CDK942 (green oval) and CDK955 (orange oval) can separately bind to individual CycT1 (yellow circle), CycT2a (violet oval), CycT2b (lavender oval) and CycK (pink oval). B) Inactive P-TEFb complexes with CycT1. Only large complexes with CycT1 are presented for illustration, but the same would apply for CycT2a, CycT2b and CycK too. Complexes of 42- and CDK955 with CycT1 are presented at the left side. 'Large' complexes consisting of CDK9/CycT1 are at the right side. Hexim1/7SK snRNA (light blue oval), Hexim2/7SK snRNA (turquoise blue oval), MEPCE/LARP7 (light orange oval).