The insulin-like growth factor-I receptor (IGF-IR) pathway is required for the maintenance of the transformed phenotype in neoplastic cells and hence has been the subject of intensive drug discovery efforts. A key aspect of successful clinical development of targeted therapies directed against IGF-IR will be identification of responsive patient populations. Toward that end, we have endeavored to identify predictive biomarkers of response to an anti-IGF-IR-targeting monoclonal antibody in preclinical models of breast and colorectal cancer. We find that levels of the IGF-IR itself may have predictive value in these tumor types and identify other gene expression predictors of in vitro response. Studies in breast cancer models suggest that IGF-IR expression is both correlated and functionally linked with estrogen receptor signaling and provide a basis for both patient stratification and rational combination therapy with antiestrogen-targeting agents. In addition, we find that levels of other components of the signaling pathway such as the adaptor proteins IRS1 and IRS2, as well as the ligand IGF-II, have predictive value and report on the development of a pathway-focused panel of diagnostic biomarkers that could be used to test these hypotheses during clinical development of IGF-IR-targeting therapies.