Miniature Schnauzers expressing this pathogenic CARD9 variant had normal numbers of T-lymphocytes and a normal CD4:CD8 ratio, but they exhibited severely suppressed T-lymphocyte and moderately impaired B-lymphocyte responses in mitogen proliferation assays44, supportive of a primary immunodeficiency that likely contributes to loss of resistance to MAC. Here, CD4 is linked to immune system disorder.