The methylation profiles of bisulfite-modified DNA of CD19+ cells from MZ twins discordant for CVID were compared using the Infinium HumanMethylation450 BeadChips (Illumina, Inc., San Diego, CA,). This platform allows the interrogation of >485,000 methylation sites per sample at single-nucleotide resolution, and comprises an average of 17 CpG sites per gene in the 99% of RefSeq genes. 96% of CpG islands are covered, with additional coverage in CpG island shores and the regions flanking them. The samples were hybridized in the array following the manufacturer’s instructions. Overall design: Total DNA isolated by standard procedures from CD19+ cells (total B lymphocytes) corresponding to two monozygotic twins discordant for common variable immunodeficiency.