Such mutations manifest as combined immunodeficiencies of varying severity, such as leaky SCID.44 One such case was recently described in a patient with a disease mimicking hyper‐IgM syndrome with high serum IgM and low IgG and IgA levels, lymphocytosis and recurrent infections, intractable diarrhoea, growth retardation, systemic cytomegalovirus infection and sclerosing cholangitis. The gene discussed is CD79A; the disease is immunodeficiency disease.