Dear Editor, Inherited muscular disorder is a heterogeneous group of genetic disorders that primarily affect skeletal muscle fibers. We have recently reported compound heterozygous mutations in ADSSL1 in two Korean families with adolescent-onset distal myopathy.1 However, ultrastructural changes associated with adenylosuccinate synthetase-like 1 (ADSSL1) myopathy have not been demonstrated previously. Here we report a third Korean family with ADSSL1 mutations and describe the ultrastructural features of ADSSL1 myopathy. A 15-year-old girl (Supplementary Fig. 1A and B in the online-only Data Supplement, II2) presented with gait disturbance. She was the second child of healthy, nonconsanguineous parents. She had first noticed difficulty running when aged 8 years, and the muscle weakness had progressed very slowly. When we examined her at an age of 15 years, she could still ambulate independently. A neurological examination revealed diffuse muscle weakness, predominantly in the leg muscles. She did not exhibit facial weakness, sensory deficits, joint contractures, or a high-arch palate. Her serum creatine kinase (CK) level was 281 IU/l (reference value <185 IU/l). The findings of needle electromyography were compatible with generalized myopathy. Her electrocardiography findings were normal. Lower-limb MRI showed subtle fatty replacement of the tibialis anterior and gastrocnemius muscles (Supplementary Fig. 1C-G in the online-only Data Supplement). The vastus lateralis muscle was biopsied when she was 15 years old. The myofibers showed marked size and shape variations along with frequent internalization of the sarcolemmal nuclei and splitting; vacuolization of the sarcoplasm, degenerating myofibers, and abnormal (whorled) arrangement of the myofibrils were also noted (Fig. 1A and B). In addition, focal endomysial fibrosis and fatty infiltration were present. Periodic acid-Schiff and Oil Red O staining did not produce abnormal positive reactions. Modified Gomori trichrome and nicotinamide adenine dinucleotide-tetrazolium reductase staining revealed whorled fibers and occasional rimmed vacuoles without ragged red fibers, cytoplasmic bodies, or nemaline rods (Fig. 1C and D). The ATPase reaction with preincubations at different pH values and immunostaining with myosin heavy chain (fast), myosin heavy chain (slow), and myosin IIa revealed a predominance of type I fibers. An electron microscopy examination showed myofibers with varying degrees of disorganization, and disorientation of the myofibrils with loss of their normal striation pattern (Fig. 1E). The myofibers showing diffuse or localized myofibrillar destruction contained remnants of the myofibrils, irregular electron-dense material of Z-line origin, and rounded and swollen mitochondria (Fig. 1F). In addition, various abnormalities of the Z-disc (including thickening) were noted, as well as displaced and replicated triads. To identify the genetic cause, exome sequencing was performed on the proband (Supplementary Table 1 in the online-only Data Supplement). Screening of myopathy-related genes revealed 27 functionally significant variants (Supplementary Table 2 in the online-only Data Supplement). We subsequently identified compound heterozygous c.910G>A (p.D340N) and Hyung Jun Park Jee Eun Lee Gyeong Seon Choi Heasoo Koo Soo Jeong Han Jeong Hyun Yoo Young-Chul Choi Kee Duk Park