Table II.

Side-by-side contrast of genome-wide association studies (GWAS) and epigenome-wide association studies (EWAS).

Scale	Genomic scale	(Epi)genomic scale
Cell lineage specificity	Genotype is static soma-wide	DNA methylation profile varies by cell/tissue type [Ohgane et al. 2008]
Age	Genotype is stable soma-wide (excepting somatic mutations)	Changes during the life course [Terry et al. 2011; Lim and Song 2012]
Gender	Differ by sex chromosomes (male: XY; female: XX)	Sex chromosomes can lead to gender bias with array-based approaches due to X-inactivation Differential methylation is reported by gender [Terry et al. 2011; Lim and Song 2012]
Race/ethnicity	Population differences in frequency of genetic polymorphisms	Limited evidence of racial variation in methylation profile [Terry et al. 2011; Lim and Song 2012]
Diet	Genotype is stable soma-wide (excepting somatic mutations)	Dietary factors can impact methyl availability [Anderson et al. 2012] Has been associated with altered methylation profile [Lim and Song 2012]
Smoking	Genotype is stable soma-wide (excepting somatic mutations)	Has been associated with altered methylation profile [Terry et al. 2011; Lim and Song 2012]
Other exogenous exposures	Genotype is stable soma-wide (excepting somatic mutations)	A host of environmental factors have been associated with altered methylation profiles [Christensen and Marsit 2011; Terry et al. 2011; Lim and Song 2012]