DNA methylation at promoters represses gene expression,and this mechanism is altered in CF nasal epithelial cells, bloodcells, and lung macrophages.41 However,it is still unclear whether this modification is a trigger or a resultof the disease.42 CFTR expression is regulatedby a housekeeping promoter and distal cisregulatory elements.43 Comparison of fetal and adult tissues of lung,colon, and intestine revealed that CFTR expression was reduced duringdevelopment as a result of DNA methylation. This evidence concerns the gene CFTR and cystic fibrosis.