The Notch pathway plays a central role in embryonic development, the regulation of stem and progenitor cells, and is implicated in many human cancers. Notch1 is also known to be activated by oncogenic mutations in tumors including T cell leukemia, chronic lymphocytic leukemia and non-small cell lung cancer. Here we report the discovery of similar mutations in breast cancer. We have developed a primary human xenograft model from a patient bearing an activating Notch1 mutation and have shown that blocking Notch signaling in this model with a novel anti-Notch1 antibody, OMP-52M51, impeded tumor growth and reduced cancer stem cell (CSC) frequency. OMP-52M51 is a humanized antibody that binds with high affinity and selectivity to human Notch1 and antagonizes Notch signaling. Additionally, we sought to determine the frequency of Notch pathway activation across a large panel of human tumors (n>600) by an immunohistochemistry assay that detects the activated form of Notch1 using a polyclonal antibody that specifically recognizes the Notch1 intracellular domain (ICD). Using this test and a rigorous H-score cut-off of 30, we found elevated Notch1 in 7-29% of the following cancers: chemo-resistant breast, gastric, cholangiocarcinoma, esophageal, hepatacellular carcinoma (HCC), and small cell lung cancer (SCLC). The class of patients that showed the highest frequency of elevated Notch1.ICD was chemo-resistant breast cancer (∼30%). This frequency was significantly higher than in unselected breast cancer patients, suggesting that Notch1 signaling plays a significant role in breast cancer chemoresistance. Interestingly, the human xenograft model which showed sensitivity to OMP-52M51 was derived from a patient that failed to respond to pre-operative chemotherapy and developed metastatic disease following surgery. Thus, chemo-resistant breast cancer as well as the other tumors with high Notch1.ICD may represent important classes of patients who could benefit from anti-Notch1 therapy in the clinic. Citation Format: Belinda Cancilla, Jennifer Cain, Min Wang, Lucia Beviglia, Jalpa Shah, Austin Gurney, John Lewicki, Laura Esserman, Tim Hoey, Ann M. Kapoun. Anti-Notch1 antibody (OMP-52M51) impedes tumor growth and cancer stem cell frequency (CSC) in a chemo-refractory breast cancer xenograft model with an activating Notch1 mutation and screening for activated Notch1 across multiple solid tumor types. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3728. doi:10.1158/1538-7445.AM2013-3728