MT-TL1 and cardiomyopathy: The m.4277T>C and m.4300A>G MTTI gene mutations cause isolated cardiomyopathy and are found at homoplasmic levels (Taylor et al, 2003; Perli et al, 2012), whereas the m.3243A>G MTTL1 gene mutation is heteroplasmic, and can be associated with severe biochemical and clinical phenotypes, such as those ascribed to MELAS (Kaufmann et al, 2011) at high mutation loads.