The Notch family of transmembrane receptors encompasses a central pathway of cell-cell contact-dependent signaling in cells both in development and cancer. In breast cancer hyperactivity of Notch is linked to aggressive therapy-resistant forms. Human clinical trials of Notch pathway inhibitors are ongoing and several approaches to intervene with the pathway are under preclinical evaluation. To date there is no approved Notch targeted therapy in the clinic. The challenges are prominent side effects, lack of specificity and poor knowledge of the underlying mechanisms behind deregulated Notch activity. However, the study of post-translational modifications of the Notch receptors has the potential to unravel as of yet unknown regulatory pathways of high importance to Notch signaling outcomes, and open up for novel cancer therapies. We are applying for a research grant from the Jane & Aatos Erkko Foundation to study Pim kinase-mediated regulation of the Notch3 paralog in breast cancer utilizing the CRISPR technology, a novel gene editing tool which allows us to introduce specific regulatory mutations in the Notch3 gene. We have shown that the intracellular domains of Notch1 (NICD1) and Notch3 (NICD3) are phosphorylated by oncogenic Pim kinases. Here we wish to study the physiological importance of the phosphorylation of NICD3. Notch3 activation in breast cancer is known to be linked to inflammatory breast cancer and brain metastases of breast cancer. We have established that the phosphorylation site is within the important RAM-domain, which is necessary for binding of NICD3 to CSL, the Notch transcriptional regulator. Furthermore, phosphorylation perturbs the binding of NICD3 to CSL, affecting p21 levels with the potential to fine-tune the observed tumor promoting or tumor suppressing effect of Notch3. The knowledge acquired from this research could shed new light on the development of novel therapeutic regimes and combinatorial treatment tactics.