ICL was first characterized in the early 1990 s and is a primary immune disorder of CD4 T cell lymphocytopenia (less than 300 cells/microL or less than 20% of lymphocytes), which is not due to any known infectious process, exogenous medication, autoimmune cytopenia, or other underlying disorder associated with lymphocytopenia. ICL patients are at risk for a wide spectrum of opportunistic and other serious infections, autoimmune disorders, and other types of lymphocytopenia. At present, no validated treatment exists for ICL, and treatment is directed primarily toward infectious complications once they arise. A first-generation form of rhIL-7 was shown in pre-clinical and Phase I studies in oncology and human immunodeficiency virus (HIV)-infected patients to be well tolerated in repeated dose trials, with long-lasting increases in both CD4 and CD8 T cells. CYT107 is a second-generation rhIL-7 product made by Cytheris via a recombinant mammalian cell culture system.