Mitochondrial cytopathies are genetic diseases, some of which result from mutations affecting part of the multiple DNA molecules located within mitochondria (mtDNA). Coexistence of mutant and wild-type mtDNA molecules is called “heteroplasmy” and defines a “mutant load”. Disease symptoms occur when the mutant load surpasses a tissue-specific threshold.

Owing to the absence of therapy and the high recurrence risk for these serious diseases, at-risk couples commonly request a prenatal diagnosis, based on determination of the mutant load on a single chorionic villous sample (CVS). We have shown that this mutant load is widely heterogeneous within placentas, so that mutant load determined from CVS should be interpreted with caution and associated to/substituted by a mutant load measurement on amniocytes. (By Drs. Julie Steffann and Jean-Paul Bonnefont, http://jmg.bmj.com/content/early/2017/07/27/jmedgenet-2017-104615 )