In most cell signaling experiments, analytes are measured one Western blot lane at a time in a semiquantitative and often poorly specific manner, limiting our understanding of network biology and hindering the translation of novel therapeutics and diagnostics. We show the feasibility of using multiplex immuno-MRM for phospho-pharmacodynamic measurements, establishing the potential for rapid and precise quantification of cell signaling networks. A 69-plex immuno-MRM assay targeting the DNA damage response network was developed and characterized by response curves and determinations of intra-and interassay repeatability. The linear range was >3 orders of magnitude, the median limit of quantification was 2.0 fmol/mg, the median intra-assay variability was 10% CV, and the median interassay variability was 16% CV. The assay was applied in proof-of-concept studies to immortalized and primary human cells and surgically excised cancer tissues to quantify exposure-response relationships and the effects of a genomic variant (ATM kinase mutation) or pharmacologic ( Because there is limited correlation between mRNA and protein levels/activity (1), quantification of proteins and posttranslational modifications is critical to understanding cellular signaling and determining pharmacodynamic (PD) 1 responses. Phosphorylation is a key post-translational modification used in signaling networks to modulate protein/pathway activity, protein interactions, and protein localization in response to extracellular and intracellular stimuli. Many diseases exhibit dysfunctions in signaling networks, and thus major efforts to identify novel drug targets (e.g. kinase inhibitors) are based on signal transduction pathways (2).Currently the research community lacks high throughput, quantitative tools for studying phospho-signaling networks, hindering our basic understanding of network biology and hence the translation of novel therapeutics and companion diagnostics. In most experiments, one analyte is measured one Western blot lane at a time in a semiquantitative and often nonspecific manner. These drawbacks limit our ability to extend knowledge beyond individual phosphorylation events to a system-wide study of phosphorylation dynamics, which is critical because signal transduction pathways act as interconnected networks, and the effects of mutations in individual genes (as well as the effects of pharmacologic compounds) spread throughout the network (3). Although Western blotting and related traditional immuno-assay platforms (e.g. ELISA) have been pushed brilliantly to their limits and have formed the basis of many advances in biomedical research, they are inadequate to support the needs of the postgenomic world, in which we need innovative technologies for determining the effects of any experimental condition (e.g. agonist or antagonist exposures, genetic variations) on the major signal transduction networks of the human cell, using precise, standardized, moderate-to-high throughput methods that can be reproduced across laboratories.Newer tec...