Cyclosporine has been isolated from the soil fungi Cylindrocarpum lucidum and Tolyplocadium inflatum. It primarily affects the T cell immune response by blocking production of interleukin 2 (IL2). It also inhibits gene transcription for interferon-, IL-3, IL-4, c-myc, c-fos and other genes required for the differentiation and proliferation of T and B lymphocytes. Like corticosteroids, cyclosporine blocks the progression of activated T cells through the cell cycle by preventing entry into the S phase.