HSPG2 and Schwartz-Jampel syndrome: Although SJS was conventionally classified into types 1a, 1b, and 2 according to clinical severity [2], it was later clarified that type 1 is caused by mutations in heparan sulfate proteoglycan 2 (HSPG2) [3] and type 2 by mutations in the leukemia inhibitory factor receptor [4]; thus, types 1 and 2 have been regarded as distinct diseases.