[Neonatal hypotonias with congenital disproportion of various types of muscular fiber, especially type I fibers. Demonstration of the familial character of this new entity].

Two sisters presenting with benign congenital hypotonia are reported. In both cases the muscle biopsies demonstrated the same pathological pattern, consisting in an abnormal size disproportion between the two main cytoenzymological types of muscle fibers. Their father, exhibiting a slight and diffuse muscle weakness, showed a closely related histological aspect. These three cases bring the first evidence of a familial transmission of this new entity. Its relationship with the other types of "congenital myopathies" is discussed.