We read with great interest the article by Roefs and colleagues on the use of orbicularis oculi muscle biopsies sampled at the time of ptosis surgery to reach a molecular diagnosis for patients exhibiting classical features of chronic progressive external ophthalmoplegia (CPEO).1 All three cases included in this report were sporadic and no additional neurological features were present. Histochemical analysis of the orbicularis oculi muscle revealed pathological hallmarks suggestive of an underlying mitochondrial disease process with ragged red fibres and isolated clusters of cytochrome c oxidase (COX)-deficient fibres. Long-range PCR analysis was performed on extracted orbicularis oculi muscle DNA and single, large-scale mitochondrial DNA (mtDNA) deletions of varying sizes were identified in all three patients, confirming the clinical diagnosis. We fully agree with Roefs and colleagues that orbicularis oculi muscle, if available, can prove a useful tissue for …