Pharmacodynamic (PD) biomarkers can measure the direct interaction between a drug and its receptor.  Pharmacodynamic biomarkers reveal drug mechanisms, if the drug has its intended effect on the biology of the disease, ideal biological dosing concentrations, and physiologic response/resistance mechanisms. Pharmacodynamic biomarkers are particularly relevant in drug mechanisms of tumor cells, where pharmacodynamic endpoints for drug interventions can be assessed directly on tumor tissues.  For example, protein phosphorylation biomarkers indicate alterations in target protein kinases and activation of downstream signaling molecules.