Immunotherapy received prior to serum sampling is a plausible explanation for the cavin-4 IgG seronegativity in patients with iRMD, both of whom lacked mutations in CAV3 and CAVIN1. For patient 9, the subsequent resolution of symptoms, the muscle biopsy’s mosaic pattern of caveolin-3 loss, and the detection of cavin-4–binding IgG in the serum by phage immunoprecipitation sequencing (but not by other methods) strongly supported an autoimmune pathogenesis. Here, CAVIN1 is linked to Autoimmunity.