Namely, even though depression-concordant MZ pairs show considerable symptom similarity,25 previous research has shown an important role for unique environmental differences in fostering psychopathological heterogeneity within these MZ twin pairs.58, 59 Thus, it is also worth noting that genes with large DNA methylation intrapair differences across diagnostic-concordant pairs were enriched for ‘nervous system development', by the interaction of protocadherin members and ZNF423 (Table 3 and Figure 2). This evidence concerns the gene PCDH11X and major depressive disorder.