The authors dedicate this article to Prof. Chris Marshall, FRS (1949–2015), an inspirational scientist whose work contributed enormously to our understanding of the RAS-regulated RAF–MEK–ERK pathway. The ERK signalling pathway is activated by an array of receptor types, including receptor tyrosine kinases (RTKs), G protein-coupled receptors and cytokine recep- tors, and the core components of this pathway are now well known 1,2 . Activated RTKs recruit adaptor proteins and guanine nucleotide exchange factors (GEFs; such as SOS) to activate the HRAS, KRAS or NRAS GTPases at the inner leaflet of the plasma membrane (FIG. 1). Once activated, GTP-bound RAS (RAS–GTP) drives the for- mation of high-activity homodimers or heterodimers of the RAF protein kinases (ARAF, BRAF or CRAF), which directly phosphorylate and activate MEK1 and MEK2 (also known as MAPKK1 and MAPKK2). MEK1 and MEK2 are dual-specificity kinases that activate ERK1 and ERK2 by phosphorylating them at conserved threonine and tyrosine residues in the T-E-Y motif found in their activation loop. Hundreds of proteins have been defined as ERK1 and ERK2 substrates and ERK-interacting partners 1,3 ; these include other pro- tein kinases and transcription factors (such as ETS and the activator protein 1 complex (AP1)), which regulate the expression of immediate- and delayed-early genes such as the D-type cyclins to promote G1/S progression in the cell cycle 4 . ERK1 and ERK2 can also regulate cell survival by phosphorylating members of the apoptosis regulating BCL-2 protein family at the mitochondria 5 . ERK1/2 signalling regulates processes that are crucial for normal development, including cell proliferation, dif- ferentiation, survival and cell motility; indeed, germline deletion of some components of the ERK pathway causes embryonic lethality 6 , and a MEK1/2 inhibitor (MEKi) forms part of the 2i protocol that maintains embryonic stem cell pluripotency 7 . The same cellular processes are deregulated in cancer and represent some of the key hall- marks and driving characteristics of the cancer cell 8,9 . Many human cancers contain activating mutations in genes encoding RTKs, RAS, BRAF, CRAF, MEK1 or MEK2, which act as driving oncogenes; consequently, many cancers exhibit deregulated activation of, and an enhanced dependency on, ERK1/2 signalling. The discovery of the core components of the RAS– ERK pathway 2 kick-started a protein kinase drug dis- covery effort that continues today 10–12 . The first ERK pathway inhibitor to be discovered, PD98059, was reported 20 years ago 13 and was shown to act inde- pendently of ATP as an apparent allosteric inhibitor of MEK1 and MEK2. Since then, MEKis have proved to be 1 Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK. 2 Signalling Laboratory, The Babraham Institute, Babraham Research Campus, Cambridge CB22 3AT, UK. 3 AstraZeneca, Oncology iMed, Cancer Biosciences, Cancer Research UK, Li Ka Shing Centre, Cambridge Institute, Robinson Way, Cambridge CB2 0RE, UK. Correspondence to P.D.S. and S.J.C. e-mails: paul.d.smith@ astrazeneca.com; simon.cook@babraham.ac.uk doi:10.1038/nrc4000 2i A cocktail of two protein kinase inhibitors, one inhibiting MEK1 and MEK2, and the other inhibiting glycogen synthase kinase 3 (GSK3). MEK1 and MEK2 inhibitors and cancer therapy: the long and winding road Christopher J. Caunt 1 , Matthew J. Sale 2 , Paul D. Smith 3 and Simon J. Cook 2 Abstract | The role of the ERK signalling pathway in cancer is thought to be most prominent in tumours in which mutations in the receptor tyrosine kinases RAS, BRAF, CRAF, MEK1 or MEK2 drive growth factor-independent ERK1 and ERK2 activation and thence inappropriate cell proliferation and survival. New drugs that inhibit RAF or MEK1 and MEK2 have recently been approved or are currently undergoing late-stage clinical evaluation. In this Review, we consider the ERK pathway, focusing particularly on the role of MEK1 and MEK2, the ‘gatekeepers’ of ERK1/2 activity. We discuss their validation as drug targets, the merits of targeting MEK1 and MEK2 versus BRAF and the mechanisms of action of different inhibitors of MEK1 and MEK2. We also consider how some of the systems-level properties (intrapathway regulatory loops and wider signalling network connections) of the ERK pathway present a challenge for the success of MEK1 and MEK2 inhibitors, discuss mechanisms of resistance to these inhibitors, and review their clinical progress. REVIEWS NATURE REVIEWS | CANCER VOLUME 15 | OCTOBER 2015 | 577 © 2015 Macmillan Publishers Limited. All rights reserved