Type I IFNs (IFN-α, IFN-β, and IFN-ω) are produced in most cell types upon virus infection and upon stimulation by dsRNA (with exceptions of embryonal carcinoma cells which are defective in expressing IFN genes). In addition, certain growth factors and lymphokines such as IL-1, colony stimulating factor (CSF), and TNF also induce type I IFNs. Both IFN-α and IFN-β are capable of enhancing induction of type I IFN genes. Protein synthesis inhibitors such as cycloheximide often enhance levels of IFN mRNA, perhaps by inhibiting a labile repressor molecule. Type II IFN (IFN-γ) is produced in T cells (TH1) and NK cells in response to stimulation with lymphokines such as IL-2 and IL-12. IFN-γ is also induced by antigen stimulation through the T cell receptor/CD3 as well as by a costimulator CD28. Its production is down-regulated by IL-4.