Further studies on larger datasets are needed to determine whether lower-penetrance PMS2 variants must be coinherited with another pathogenic allele to cause CMMRD, similar to the genetic model recently described in Fanconi anemia (FA) (15), in which one BRCA1 or BRCA2 variant retains partial protein function to ensure embryonic viability. The gene discussed is BRCA2; the disease is Friedreich ataxia.