Immunodeficiency 31A is a rare autosomal dominant disorder characterized by a partial defect in the interferon (IFN)-gamma pathway, leading to mild mycobacterial infections such as Bacillus Calmette-Guerin (BCG) vaccine, environmental non-tuberculous mycobacteria, and by the more virulent Mycobacterium tuberculosis. Disease severity depends on the degree of impairment of interferon-gamma mediated immunity.  Some patients die of overwhelming mycobacterial disease with lepromatous-like lesions in early childhood, whereas others develop, later in life, disseminated but curable infections with tuberculoid granulomas.

Immunodeficiency 31A is caused by heterozygous mutations in the STAT1 gene on chromosome 2q32.2, encoding the signal transducer and activator of transcription 1. Two distinct forms have been described; one affecting phosphorylation and the other impairing DNA-binding activity.