The invention provides, among other aspects, compositions and methods for treating, preventing, and diagnosing diseases or conditions associated with an abnormal level or activity of biglycan; diseases or conditions associated with an abnormal level or activity of collagen VI; disorders associated with an unstable cytoplasmic membrane, due, e.g., to an unstable dystrophin associated protein complex (DAPC); and disorders associated with abnormal synapses or neuromuscular junctions, including those resulting from an abnormal MuSK activation or acetylcholine receptor (AChR) aggregation.