The PWWP domain is an essential component of DNA methyltransferase 3 B (Dnmt3b) which is responsible for establishing DNA methylation patterns during embryogenesis and gametogenesis. In tumorigenesis, DNA methylation by Dnmt3b is known to play a role in the inactivation of tumor suppressor genes. In addition, a point mutation in the PWWP domain of Dnmt3b has been identified in patients with ICF syndrome (immunodeficiency, centromeric instability, and facial anomalies), a rare autosomal recessive disorder characterized by hypomethylation of classical satellite DNA. The PWWP domain, named for a conserved Pro-Trp-Trp-Pro motif, is a small domain consisting of 100-150 amino acids. The PWWP domain is found in numerous proteins that are involved in cell division, growth and differentiation. Most PWWP-domain proteins seem to be nuclear, often DNA-binding, proteins that function as transcription factors regulating a variety of developmental processes.