Siddiqui and colleagues demonstrated that HEXIM1 (also known as Cardiac Lineage Protein-1 or CLP-1) knockout in mice is lethal in late fetal stages due to heart failure, and that bigenic mice overexpressing CycT1 in a HEXIM (+/-) heterozygote background exhibit enhanced susceptibility to cardiac hypertrophy accompanied by elevated Cdk9 activity [193,194]. Here, HEXIM1 is linked to heart failure.