The affinity of LDC4297 for CDK7 proves to be extremely high. Kinase assays performed for CDK1, CDK2, CDK4, CDK6, CDK7, and CDK9 confirms the selective inhibitory activity of LDC4297 for CDK7 in the nano-picomolar range (IC50, 0.13±0.06 nM for CDK7 versus IC50s between 10 nM and 10,000 nM for all other analyzed CDKs). LDC4297 exerts anticytomegaloviral activity. Human cytomegalovirus (HCMV) replication is inhibited by LDC4297 in a concentration-dependent manner with an EC50 value of 24.5±1.3 nM. Inhibition is statistically significant and morphological signs of cytotoxicity only occurrs at concentrations of 3.3 μM or higher. Anti-HCMV activity of LDC4297 is exerted through a multifaceted mode of action that involves an interference with virus-induced Rb phosphorylation. Virus replication is broadly blocked by LDC4297, whereby the antiviral efficacies varied between the viruses used, i.e., strong efficacy for HSV-1 and VZV (EC50s = 0.02 and 0.06 μM, respectively) and intermediate to low efficacy for HSV-2 and EBV (EC50s = 0.27 and 1.21 μM, respectively.