Complete interleukin-12/interleukin-23 receptor β1 (IL-12Rβ1) deficiency is the most frequent known genetic etiology of the syndrome of Mendelian susceptibility to mycobacterial disease. The patients described to date lack IL-12Rβ1 at the surface of their natural killer (NK) and T cells due to IL12RB1 mutations, which either interrupt the open reading frame or disrupt protein folding. We describe a patient with a large in-frame deletion of 12165 nucleotides (nt) in IL12RB1, encompassing exons 8 to 13 and resulting in the surface expression of nonfunctional IL-12Rβ1. These 6 exons encode the proximal NH₂-terminal half of the extracellular domain downstream from the cytokine-binding domain. Five of 6 monoclonal anti-IL-12Rβ1 antibodies tested recognized the internally truncated chain on the cell surface. However, IL-12 and IL-23 did not bind normally to the patient's IL-12Rβ1-containing respective heterodimeric receptors. As a result, signal transducer and activator of transcription-4 (STAT4) was not phosphorylated and interferon-γ (IFN-γ) production was not induced in the patient's cells upon stimulation with even high doses of IL-12 or IL-23. The functional defect was completely rescued by retrovirus-mediated IL-12Rβ1 gene transfer. Thus, the detection of IL-12Rβ1 on the cell surface does not exclude the possibility of complete IL-12Rβ1 deficiency in patients with mycobacteriosis or salmonellosis. Paradoxically, the largest IL12RB1 mutation detected is associated with the cell surface expression of nonfunctional IL-12Rβ1, defining a novel genetic form of IL-12Rβ1 deficiency. (Blood. 2004;104:2095-2101)