Point mutations in the phosphorylation domain of the Bcr-Abl fusion oncogene give rise to drug resistance in chronic myelogenous leukemia patients. These mutations alter kinase-mediated signaling function and phenotypic outcome. An information theoretic analysis of the correlation of phosphoproteomic profiling and transformation potency of the oncogene in different mutants is presented. The theory seeks to predict the leukemic transformation potency from the observed signaling by constructing a distribution of maximal entropy of site-specific phosphorylation events. The theory is developed with special reference to systems biology where high throughput measurements are typical. We seek sets of phosphorylation events most contributory to predicting the phenotype by determining the constraints on the signaling system. The relevance of a constraint is measured by how much it reduces the value of the entropy from its global maximum, where all events are equally likely. Application to experimental phospho-proteomics data for kinase inhibitor-resistant mutants shows that there is one dominant constraint and that other constraints are not relevant to a similar extent. This single constraint accounts for much of the correlation of phosphorylation events with the oncogenic potency and thereby usefully predicts the trends in the phenotypic output. An additional constraint possibly accounts for biological fine structure.high-throughput measurements | information theory | phospho proteomics | signal transduction networks | systems biology B iological systems use complex networks of molecular events, such as signaling and transcription, to regulate cellular outcome. Experimental biology is now able to measure up to thousands of these events from individual samples, inspiring efforts to identify both the events most contributory to cellular phenotypes and the nature of how multiple regulatory mechanisms are coordinated. Such understanding is crucial to the next generation of single agent and mixture molecularly targeted therapeutics. Kinase-mediated signaling is central to the execution of cellular programs and to communication with the environment and other cells. Aberrant signaling is implicated in many diseases and is a hallmark of cancer (1). Mass spectrometry-and antibody-based phospho proteomics allow the global profiling of the state of the signaling network (2-5). These approaches permit site-specific monitoring of phosphorylation and, thus, provide discrimination of the sometimes multiple and differential regulatory phosphorylation events on individual proteins. Here, we develop and apply an information theoretic maximal entropy analysis of the correlation of signaling events to cellular outcome to identify the dominant constraints that describe and predict the phenotypic output.There are equivalent ways of motivating the choice of a distribution of maximal entropy (6). From an information theoretic point of view it is the distribution that is consistent with the data at hand and is otherwise least informative. From ...