Project Summary/Abstract Approximately half of patients with malignant melanoma and papillary thyroid cancer (PTC) have a tumor that harbors a BRAFV600 mutation. Mutation detection is the critical branch-point in determining whether BRAF-targeted therapy is an option for patients with advanced (unresectable Stage III and IV) melanoma. Also, knowing a PTC patient's BRAF status may be clinically actionable, since it may guide the extent of initial surgery (lobectomy versus total thyroidectomy and consideration of central lymphadenectomy), the approach to surveillance imaging (radioactive iodine scan versus PET-CT), and adjuvant therapy. Currently, commercial BRAF mutational analysis assays are limited by sensitivity and lack robust clinical validation in large numbers of patients treated with a BRAF inhibitor or following tumor resection in the high-risk setting. With BRAF targeted therapy now established as a standard therapy for patients with BRAF mutant malignancies, we feel that the development of highly-sensitive, blood-based assays have the potential to greatly improve the care of patients. We have formerly described testing with a highly sensitive assay that has the ability to detect BRAFV600 mutations in the blood of patients with BRAF mutant melanoma, and now have shown that this assay has excellent sensitivity and specificity vis--vis tissue based analysis in both melanoma and PTC patients. In this proposal, we aim to optimize our assay in the research laboratory at Beth Israel Deaconess Medical Center, where all the previous work with the assay has been performed, during the UH2 portion of the grant and harmonize the assay in a CLIA-approved laboratory at Massachusetts General Hospital (MGH). Once the assay has been optimized, harmonized and successfully transferred to the CLIA- approved laboratory at MGH, we plan to clinically validate our assay in patients with Stage III and Stage IV melanoma and resectable and metastatic PTC through the analysis of samples obtained from several different clinical trials. These include a neoadjuvant trial of dabrafenib plus trametinib in patients with Stage III BRAF-mutant melanoma (NCT02231775), a randomized Phase III study of the sequence of dabrafenib plus trametinib followed by ipilimumab plus nivolumab compared with the other sequence in treatment nave patients with advanced melanoma (NCT02224781), a Phase III randomized double blind study comparing the complete remission rate following a 4-week course of the MEK inhibitor, selumetinib, or placebo and a single dose adjuvant radioactive iodine therapy in patients two phase II targeted therapies, one with the multi-tyrosine kinase inhibitor lenvatinib (NCT02657369) and the other with the mTOR inhibitor MLN0128 (NCT02244463). with differentiated thyroid cancer (NCT02393690), and