We randomly assigned 418 previously untreated patients who had metastatic melanoma without a BRAF mutation to receive nivolumab ( at a dose of 3 mg per kilogram of body weight every 2 weeks and dacarbazine-matched placebo every 3 weeks ) or dacarbazine ( at a dose of 1000 mg per square meter of body-surface area every 3 weeks and nivolumab-matched placebo every 2 weeks ) .