The invention provides compositions and methods for treating, preventing, and diagnosing diseases or conditions associated with an abnormal level or activity of biglycan; disorders associated with an unstable cytoplasmic membrane, due, e.g., to an unstable dystrophin associated protein complex (DAPC); disorders associated with abnormal synapses or neuromuscular junctions, including those resulting from an abnormal MuSK activation or acetylcholine receptor (AChR) aggregation. Examples of diseases include muscular dystrophies, such as Duchenne's Muscular Dystrophy, Becker's Muscular Dystrophy, neuromuscular disorders and neurological disorders.