In this section, we review two examples of immunodeficiencies known to display alterations in DNA methylation: Immunodeficiency-centromeric instability-facial anomalies (ICF) syndrome, a monogenic PID characterized by mutations of the DNMT3B gene, and common-variable immunodeficiency (CVID), a complex disease with no common causative gene. The gene discussed is DNMT3B; the disease is Immunodeficiency.