Likewise, Dnmt3b-deficient bone marrow cells can reconstitute long-term hematopoiesis, but they have reduced B cells, mimicking what is seen in human patients with the “immunodeficiency, centromeric instability, and facial dysmorphism syndrome, type I” [Online Mendelian Inheritance in Man (OMIM), no. 242860] (1, 21, 22), often caused by loss-of-function mutations in the DNMT3B gene. This evidence concerns the gene DNMT3B and immune system disorder.