DNMT3B and immunodeficiency-centromeric instability-facial anomalies syndrome 1: In addition, biallelic DNMT3B mutations in Immunodeficiency, Centromeric instability, and Facial anomalies syndrome 1 (ICF1) patients, most of which are loss of function variants in the C-terminal catalytic domain of DNMT3B, are also associated with severe D4Z4 hypomethylation.