Among the 1.6 million women diagnosed with breast cancer every year, 10-16% develop brain metastases who have a devastating <20% one-year survival. At present, no effective drug treatment exists for patients with breast cancer brain metastasis. Therefore, effective therapies are urgently needed for this population. Unfortunately, developing effective therapies for brain metastasis is largely hampered by a lack of in-depth understanding of the biological mechanisms of brain metastasis, which could guide drug development and clinical trials. To surmount this challenge, we have performed an in vivo human kinome screen to uncover novel kinases that promote breast cancer brain metastasis in mice. Kinases are at the central nodes of cancer cell signaling networks critical for cancer progression and metastasis, and can serve as druggable therapeutic targets. Among them, cyclin-dependent kinase 5 (CDK5) was a top “hit”, with >26-fold enrichment. CDK5, a non-canonical CDK, can increase cell proliferation, migration, and angiogenesis. CDK5 overexpression (+++) significantly correlated with high grade breast cancer and lower survival in patients, but its function in brain metastasis was overlooked. Thus, we further examined CDK5 function in brain metastasis and found that CDK5+++ cells a) confer increased brain metastasis and decreased survival in mice; b) can adapt to, and proliferate in, low glucose and hypoxic culture condition that mimics the environment in the brain; c) induce immunosuppression by down-regulating MHC class I (MHC-I). Conversely, mice-bearing brain metastases of CDK5 knockdown tumor cells had prolonged survivals. Attractively, CDK5 is readily targetable with clinically- applicable, blood-brain-barrier penetrating inhibitors. Here, we hypothesize that CDK5 activation promotes breast cancer brain metastasis by both facilitating cancer cell adaptation/outgrowth in the brain and by immunosuppression; CDK5 inhibitors may be developed as new therapeutics for breast cancer brain metastasis. The major goals of this proposal are 1) Determine the brain metastasis-promoting functions of CDK5 in spontaneous brain metastasis models and in immune competent mouse models, and examine its clinical relevance in patient specimens; 2) Explore novel mechanisms of CDK5-enhanced brain metastasis by examining the impact of CDK5+++ in cancer cells on both a) breast cancer cell adaptation/outgrowth in the brain and b) inducing immune suppression; 3) Evaluate the potential of targeting CDK5 for early intervention and treatment of brain metastasis to prolong mouse survival. The successful completion of these studies will bring about new understanding of breast cancer brain metastasis and the first generation of effective brain metastasis-targeted therapies. Ultimately, our findings will be translated to clinical trials, leading to new and better treatments for breast cancer patients having brain metastasis.