Brugha R, et al. – This study was meant to compare the pattern of deoxyribonucleic acid (DNA) methylation in blood, buccal and nasal epithelial cells to that in airway epithelial cells from children. Investigations demonstrated that DNA methylation in blood poorly reflects methylation in airway epithelium. Further research work should consider measurement of methylation either in buccal cells or, preferably, in nasal epithelial cells.