On May 23, 2017, the US Food and Drug Administration (FDA) granted accelerated approval to pembrolizumab (Keytruda, Merck & Co) for patients with unresectable or metastatic, microsatellite instable-high (MSI-H) or mismatch repair-deficient (dMMR) solid tumors that have progressed on prior therapy and have no satisfactory treatment options. This approval is, to date, the first drug authorized for use based on a molecular biomarker rather than a traditional histopathologic diagnosis. The approval bridges several themes in modern oncologic drug development: the rapid translation of high-quality basic science, tailoring therapy based on genetic subgroups, and a flexibility on the part of the FDA to authorize drugs based on early, favorable data.