Data from four published datasets were analyzed to determine whether preexisting MEK1P124 mutations affect radiologic response or progression-free survival (PFS) in patients with BRAFV600-mutant metastatic melanoma treated with vemurafenib or dabrafenib...In a pooled analysis of 123 patients, the presence of a pretreatment MEK1P124 mutation (N = 12, 10%) was associated with a poorer RECIST response (33% vs. 72% in MEK1P124Q/S vs. MEK1P124 wild-type, P = 0.018), and a shorter PFS (median 3.1 vs. 4.8 months, P = 0.004).