Advances in diagnostic methods have also made these organisms easier to identify, and our understanding of their true global epidemiology continues to evolve, especially for rarer species such as M. haemophilum. CD4+ T lymphocytes play a central role in host defense against NTM, and their depletion, whether due to human immunodeficiency virus (HIV), idiopathic CD4+ lymphopenia, or immunosuppressive therapies, significantly increases susceptibility. The gene discussed is CD4; the disease is lymphopenia.