DNA methylation patterns are characterized by highly conserved developmental 
programs, but allow for divergent gene expression resulting from stochastic 
epigenetic drift or divergent environments. Genome-wide methylation studies in 
monozygotic (MZ) twins are providing insight into the extent of epigenetic 
variation that occurs, irrespective of genotype. However, little is known about 
the variability of DNA methylation patterns in adolescence, a period involving 
significant and rapid physical, emotional, social, and neurodevelopmental 
change. Here, we assessed genome-wide DNA methylation using the 450 K Illumina 
BeadChip in a sample of 37 MZ twin pairs followed longitudinally since birth to 
investigate: 1) the extent of variation in DNA methylation in identical genetic 
backgrounds in adolescence and; 2) whether these variations are randomly 
distributed or enriched in particular functional pathways. We also assessed 
stability of DNA methylation over 3-6 months to distinguish stable trait-like 
and variable state-like genes. A pathway analysis found high within-pair 
variability in genes associated with development, cellular mechanisms, tissue 
and cell morphology, and various disorders. Test-retest analyses performed in a 
sub-sample of 8 twin pairs demonstrated enrichment in gene pathways involved in 
organismal development, cellular growth and proliferation, cell signaling, and 
particular disorders. The variability found in functional gene pathways may 
plausibly underlie phenotypic differences in this adolescent MZ twin sample. 
Furthermore, we assessed stability of methylation over 3-6 months and found that 
some genes were stable while others were unstable, suggesting that the methylome 
remains dynamic in adolescence and that dynamic sites tend to be organized in 
certain gene pathways.