The methylation status of young Alu sequences was investigated in four ICF patients. In fibroblast and leukocyte DNAs, Alu repeats were either undermethylated (HhaI and HpaII digestion) or demethylated (BstUI digestion), in contrast with the methylated status of Alus in control subjects. The methylation profile exhibited in ICF patients reproduces the normal profile of placental or sperm DNA. High-sensitivity immunocytochemical detection of HhaI and HpaII restriction sites on metaphase chromosomes provided further evidence of this undermethylation. The DNA methylation defect in ICF patients, first detected in satellite DNAs (constitutive heterochromatin) and CpG islands of genes on the inactive X chromosome (facultative heterochromatin), thus includes Alu sequences that are widely distributed throughout the human genome.