Recent studies have established MED1 (Mediator Subunit 1, also named TRAP220, DRIP205, or MED220) as a key transcriptional coactivator for ERα in breast cancer. Significantly, the MED1 gene is located at the chromosome 17q12 region, also known as the HER2 amplicon, and co-amplifies with HER2 in nearly all instances in breast cancer. We have further confirmed MED1 overexpression and correlation with HER2 status at the protein level using human breast cancer tissue microarrays. Importantly, we found that MED1 serves as a crosstalk point for the HER2 and ERα pathways in ERα-mediated transcription. During our last funding period, we have established MED1 as a key mediator of HER2-driven tumorigenesis using our newly generated MED1 mutant knockin and MED1 overexpression mouse models. Here, we have provided further preliminary data supporting Jab1 as a key MED1 direct downstream target gene by unbiased RNA-seq analyses and further experimental validations. Interestingly, we found that Jab1 can also in turn regulate MED1 stability and is required for ER-dependent gene transcription and functions. The overall objective of this application is to elucidate the role and underlying molecular mechanism of Jab1/MED1 axis in HER2-mediated breast cancer. Our hypothesis is that the crosstalk between Jab1 and MED1 plays key roles in HER2-mediated breast cancer metastasis and treatment resistance. We have provided strong preliminary data, generated essential reagents, and assembled an outstanding team of collaborators to pursue the following specific aims: 1) elucidate the role and molecular mechanisms underlying Jab1 functions and regulations in breast cancer metastasis, 2) determine the role of MED1 turnover and its regulation by Jab1 in breast cancer metastasis, and 3) test the efficacy of targeting Jab1 and MED1 on breast cancer metastasis and treatment resistance. Through these studies, we expect to make an important positive impact not only by filling key knowledge gaps on the role of previously uncharacterized Jab1/MED1 regulatory loop in HER2+/ER+ breast cancer, but also by providing potential novel strategies for the future treatment of this difficult to treat subtype of breast cancer.