We have established an immunoassay to detect dysferlin expression on peripheral blood films (PBF) and tested its accuracy on samples obtained from healthy controls and patients with genetic diagnosis of dysferlinopathy as well as alternative genetic diagnoses of muscular dystrophy. This study will analyze 200 samples from individuals recruited in the Latin-SEQ project. Latin-SEQ is an international research collaboration coordinated by the John Walton Muscular Dystrophy Research Centre in Newcastle upon Tyne, UK, that applies targeted whole exome sequencing to >1,000 patients with undiagnosed muscle weakness from multiple centres throughout Latin America. In order to avoid faults in sample collection, which are a major cause of unsuccessful labelling, individual training sessions are arranged with each center. PBFs will then be collected from participants to the Latin-SEQ study who had onset of muscle weakness after age 15 and clinical diagnosis excluding DMD, FSHD and DM1. PBFs will be assessed for quality and dysferlin expression in neutrophils will be blind-scored. Protein data will be compared to DYSF genetic status of participants when WES result interpretation takes place through Latin-SEQ. The study will validate how the assay is likely to perform in clinical practice as an initial screening for dysferlinopathies. In addition, we will assess its suitability to large population screening and as a functional test for variants of uncertain significance in the DYSF gene.