FOS and neoplasm: In addition, IFN-α induces and maintains PD-1 expression on the TCR-engaged primary mouse T cells through the association between IFN-responsive factor 9 and the IFN stimulation response element, suggesting the role of IFN-responsive factor 9 on regulating T-cell exhaustion.42 The tumor-infiltrating T cells exhibit upregulated expression of activator protein 1 (AP-1) subunit c-Fos.