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Myeloid cell leukemia 1 (Mcl-1), a member of the Bcl-2 family of proteins, is overexpressed and amplified in various cancers and promotes the aberrant survival of tumor cells that otherwise would undergo apoptosis. Overexpression of Mcl-1 is also a resistance mechanism that prevents tumor cells from being effectively treated by existing chemotherapies. Thus, inhibition of Mcl-1 is a promising therapeutic strategy for the treatment of cancer. However, Mcl-1 exerts its effects through protein-protein interactions and is thought to be very challenging to target with small molecules. Here we describe the discovery of potent and selective Mcl-1 inhibitors using fragment-based methods and structure-based design. From an NMR-based screen of a large fragment library, over 130 hits were identified. Two distinct chemical hit series were found that bind to two different sites on Mcl-1 as revealed by NMR-derived model structures. Members of the two fragment classes were merged together to produce compounds that bind to Mcl-1 with greater than two orders of magnitude improved binding affinity. Structures of these compounds when complexed to Mcl-1 were obtained by X-ray crystallography and provide detailed information about the molecular recognition involved in small-molecule binding to Mcl-1. Lead compounds exhibited a dissociation constant of Citation Format: Taekyu Lee, Anders Friberg, Dominico Vigil, Bin Zhao, R. Nathan Daniels, Jason P. Burke, Pedro M. Garcia-Barrantes, DeMarco Camper, Brian A. Chauder, Edward T. Olejniczak, Stephen W. Fesik. Discovery of potent and selective Mcl-1 inhibitors using fragment-based methods and structure-based design. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2350. doi:10.1158/1538-7445.AM2013-2350