Primary immunodeficiencies (PIDs) were long thought to be exclusively recessive traits -- autosomal recessive (AR) in most cases, with a few X-linked recessive (XR) diseases. In recent years, autosomal dominant (AD), mitochondrial, polygenic, and even somatic PIDs have been described. However, AR remains the most frequent inheritance pattern among recently described PIDs. Some PIDs have been shown to be genetically heterogeneous. Mendelian susceptibility to mycobacterial diseases (MSMD) displays a high level of genetic heterogeneity. There are 6 MSMD-causing genes, including 1 X-linked gene (nuclear factor-kappaB-essential modulator [NEMO]) and 5 autosomal genes (IFN-gamma receptor 1 [IFNGR1], IFN-gamma receptor 2 [IFNGR2], signal transducer and activator of transcription 1 [STAT1], IL-12 p40 subunit [IL12P40], and IL-12 receptor beta-subunit [IL12RB1]). The X-linked trait is XR; STAT1 deficiency is AD; the IFNGR2, IL12P40 subunit, and IL12RB1 deficiencies are AR; and IFNGR1 deficiency may be AD or AR. Two of the AR traits (IFNGR1, IFNGR2) may be subdivided into complete and partial deficiencies, and 3 AR complete deficiencies (IFNGR1, IFNGR2, IL12RB1) may be subdivided into disorders with and without cell surface expression. Finally, there are 2 types of AD STAT1 deficiency, depending on whether the mutation impairs phosphorylation or DNA binding. Thirteen genetic disorders conferring MSMD have been described, involving 1 XR, 3 AD (2 genes), and 9 AR traits (4 genes). However, no genetic etiology has yet been identified for about half of all patients with MSMD. We expect to identify new XR and AD causes of MSMD, but new AR etiologies of MSMD are also likely to be discovered. The investigation of children from areas in which consanguineous marriages are common will probably facilitate the description of many more AR traits.