APOE and Alzheimer disease: However, we found a very strong interaction between APOE ε4 and mtDNA haplogroup K. A similar interaction has been reported by Carrieri et al for AD; this study found significant under‐representation of haplogroups K and U in ε4 carriers and concluded that these haplogroups might neutralize the harmful effect of the APOE ε4 allele.23 As illustrated by Figure 1B, in our cohort there is a strong effect of haplogroup K on outcome in patients who are APOE ε4 positive.