Summary Background Patients with melanoma harbouring Val600 BRAF mutations benefit from treatment with BRAF inhibitors. However, no targeted treatments exist for patients with BRAF wild-type tumours, including those with NRAS mutations. We aimed to assess the use of MEK162, a small-molecule MEK1/2 inhibitor, in patients with NRAS -mutated or Val600 BRAF -mutated advanced melanoma. Methods In our open-label, non-randomised, phase 2 study, we assigned patients with NRAS -mutated or BRAF -mutated advanced melanoma to one of three treatment arms on the basis of mutation status. Patients were enrolled at university hospitals or private cancer centres in Europe and the USA. The three arms were: twice-daily MEK162 45 mg for NRAS -mutated melanoma, twice-daily MEK162 45 mg for BRAF -mutated melanoma, and twice-daily MEK162 60 mg for BRAF -mutated melanoma. Previous treatment with BRAF inhibitors was permitted, but previous MEK inhibitor therapy was not allowed. The primary endpoint was the proportion of patients who had an objective response (ie, a complete response or confirmed partial response). We report data for the 45 mg groups. We assessed clinical activity in all patients who received at least one dose of MEK162 and in patients assessable for response (with two available CT scans). This study is registered with ClinicalTrials.gov , number NCT01320085 , and is currently recruiting additional patients with NRAS mutations (based on a protocol amendment). Findings Between March 31, 2011, and Jan 17, 2012, we enrolled 71 patients who received at least one dose of MEK162 45 mg. By Feb 29, 2012 (data cutoff), median follow-up was 3·3 months (range 0·6–8·7; IQR 2·2–5·0). No patients had a complete response. Six (20%) of 30 patients with NRAS -mutated melanoma had a partial response (three confirmed) as did eight (20%) of 41 patients with BRAF -mutated melanoma (two confirmed). The most frequent adverse events were acneiform dermatitis (18 60% patients with NRAS -mutated melanoma and 15 37% patients with the BRAF -mutated melanoma), rash (six 20% and 16 39%), peripheral oedema (ten 33% and 14 34%), facial oedema (nine 30% and seven 17%), diarrhoea (eight 27% and 15 37%), and creatine phosphokinase increases (11 37% and nine 22%). Increased creatine phosphokinase was the most common grade 3–4 adverse event (seven 23% and seven 17%). Four patients had serious adverse events (two per arm), which included diarrhoea, dehydration, acneiform dermatitis, general physical deterioration, irregular heart rate, malaise, and small intestinal perforation. No deaths occurred from treatment-related causes. Interpretation To our knowledge, MEK162 is the first targeted therapy to show activity in patients with NRAS -mutated melanoma and might offer a new option for a cancer with few effective treatments. Funding Novartis Pharmaceuticals.