In addition to corticosteroids, traditional immunosuppressive or chemotherapeutic agents associated with the development of PCP include various antimetabolites (methotrexate), alkylating agents (cyclophosphamide), anticalcineurins (cyclosporine and tacrolimus), and inhibitors of a mammalian target of rapamycin (mTOR) (sirolimus and everolimus) (Table 1). The gene discussed is MTOR; the disease is pneumocystosis.