CD79A and agammaglobulinemia: To decipher the mechanisms at the origin of the antibody deficiency in ICF4 syndrome, and more specifically the role of HELLS and DNA methylation during antibody responses, we describe here a mouse model in which critical exons of Hells are conditionally ablated during early B-cell development (Mb1-Cre HellsKO) or in mature naive B cells (CD21-Cre HellsKO).