Comment on: Assessment of cardiac disease in MELAS requires comprehensive, prospective work-up

To date, a specific correlation between heteroplasmy rate (HR) and degree of cardiovascular involvement has not been demonstrated and a number of additional factors are likely involved in phenotypic expression. Standard protocols for heteroplasmy detection have not been established, possibly limited by extreme intra-patient variability or HR fluctuation over time. Testing HR in multiple tissues remains undoubtedly useful for diagnostic purpose, especially in case of mild or late phenotype.