FIRE initiative targets the creation of a multidisciplinary research platform for investigation and experimental evaluation of innovative ideas in networking and services.At the infrastructure level, such a platform should embrace all major wired and wireless access technologies.FLEX aims at contributing a crucial missing piece in FIRE's infrastructure puzzle:cellular access technologies and Long-Term Evolution (LTE) in particular.LTE 4G cellular networks are gradually being adopted by all major operators in the world and are expected to rule the cellular landscape at least for the current decade. They will also form the starting point for further progress, ushering in the era of 5G mobile networks.The lack of open or at least openly configurable cellular equipment and core network software has limited applied research in this field within the boundaries of vendor and operator R&D groups.In FLEX we will build programmable LTE components as extensions to existing European testbeds, thus providing an open and remotely accessible platform for experimentation with LTE.The deployment of LTE components will take place on three existing and well-established FIRE wireless testbeds, thus reducing the required integration effort, and placing it instead on novel aspects of LTE experimentation-driven research.FLEX's experimentation environment will feature both open source platforms and configurable commercial equipment that span macro-cell, pico-cell and small-cell setups. We will build upon current FIRE testbed management and experiment control tools and extend them to provide support for the new LTE components, and we will develop specialized monitoring tools and methodologies. Focus will be placed on mobility, with the establishment of both real and emulated mobility functionalities on the testbeds.FLEX will organize two open calls, aiming to attract research groups to conduct sophisticated experiments, test innovative usages or provide functional extensions of LTE testbeds