Cellular signalling plays a central role in cancer biology.Many oncogenes and tumor suppressor target the functions of signalling networks underlying cancer malignant phenotype.The reverse phase protein microarray (RPPA) is a cell-based and antibody-orientated quantitative assay that analyzes nanoliter amounts of sample for potentially hundreds of proteins at their expression levels and modifications status, such as phosphorylation or cleavage.RPPAs allow concordant interrogation of multiple signaling molecules at their functional activity.We utilized the RPPA to profile signaling networks in breast cancer cell lines and breast tumor tissue.Linking RPPA to chemical genomics has major strengths in identification and validation of therapeutic targets, characterization of signaling networks, as well as determination of on-and off-target activity of novel drugs.