The patient is 23 year old female with past medical history of prediabetes who was diagnosed with myasthenia gravis in outside hospital presented to UCLA for evaluation of worsening bilateral ptosis. The patient was initially diagnosed at the age of 12 with myasthenia gravis at an outside hospital. Fatigue initially improved with pyridostigmine 60 mg three times a day as needed and prednisone 5 mg daily, but she ptosis never resolved. She denied diplopia. She reported some slurred speech when she is tired. Her symptoms were stable until she was 17 years old when ptosis started to get worse. On exam, noted bilateral (right > left) eyelid ptosis. The right eye had severely limited extraocular movement in all directions while the left eye showed limited abduction and adduction, which was better than the right eye. She had primarily had proximal muscle weakness on manual muscle testing. As part of the diagnostic work-up, the patient underwent antibody testing for myasthenia gravis, but acetylcholine receptor binding/blocking/modulating antibodies, MuSK antibodies, and LRP4 antibodies were negative. Creatine kinase levels were normal. Remainder of the lab work was unremarkable. During the follow-up visit, the patient underwent EMG/NCS with repetitive nerve stimulation at 3 Hx, which did not demonstrate decrement. She was subsequently referred for single-fiber EMG, which revealed clear abnormal jitter with neuromuscular blocking. Serum genetic testing was essentially negative. Muscle biopsy demonstrated subtle cytoplasmic abnormalities in mitochondrial staining. Further mitochondrial DNA testing from the biospy specimen detected heteroplasmic large deletion of m.8579_14135del5557, with a heteroplasmy level of 62.35%. The patient was ultimately diagnosed with chronic progressive external opthalmoplegia (CPEO). This case highlights the challenges of diagnosis chronic progressive external opthalmoplegia (CPEO), and how diagnostic work-up as well as clinical presentation can mimic seronegative myasthenia gravis.