The aim of this project is to gain insight in the cellular and molecular processes leading to dysfunction of the neuromuscular system in FSHD patients by large scale analysis of protein homeostasis in tissues and cell lines of patients and controls. In the past few years, projects have been launched to study deregulation of biological pathways in FSHD on RNA level. These strategies include differential display and RNA profiling experiments on commercial and custom made DNA chips and arrays. Despite their limitations, DNA arrays are now one of the most commonly used and successful methods to determine the molecular and cellular aspects of many acquired and genetic diseases. It is anticipated that also for FSHD, this approach will provide a valuable contribution in understanding its pathology. Nevertheless, protein levels, including the level of modified proteins and the composition of protein complexes are of an order of importance larger to understand FSHD pathophysiology. Consequently there is a need for protein arrays. The llama antibody technology provides a unique opportunity to develop protein arrays. The power of the llama system for this purpose is that this animal makes single (heavy) chain antibodies. Using the genetic information for this single-chain repertoire for the construction of phage-display antibody libraries abolishes the need to combine heavy- and light-chains, one of the major drawbacks of conventional phage-display libraries. Moreover, these single-chain antibodies tend to have a very high affinity and stability. It has already been demonstrated that large naive and directed libraries of antibodies can be generated. Experience in cloning, production and isolation of these llama antibodies is available. We propose to generate muscle-specific antibody arrays derived from Llama single-chain phage-display clones. To this end, a Llama will be immunized with human muscle protein homogenates, and after peak response, a phage display library will be constructed. Antibody clones will be selected with a variety of selection procedures (e.g. with recombinant proteins or with muscle homogenates from different species) and arrayed on glass slides. Well characterized single chain antibody arrays will be used to study FSHD pathophysiology on fluorescently labeled protein homogenates of tissues and cell cultures of patients and controls. Evidently, these antibodies can also be used individually for specific immunohistochemical and immunocytochemical studies.