Muscle weakness associated to marked joint deformities is not an uncommon clinical situation in daily neuromuscular clinics. These abnormalities encompass a large variety of conditions including non-primary muscle disorders. Besides well-defined and rather readily recognisable hereditary syndromes such as Bethlem myopathy or Ullrich congenital muscular dystrophy, some unusual etiologies should also be considered. We report here two paradigmatic cases in which we found mutations in two novel genes corresponding to two newly described entities (progressive pseudorheumatoid dysplasia, PPD, and infantile systemic hyalinosis, ISH) both conditions in which the clinical picture can mimick primary muscle disease.