A radiation-inducible immediate-early gene, IEX-1, was identified and characterized in human squamous carcinoma cells. Sequence analysis revealed 156-amino acid nucleotides, encoding a protein of Mr 20,000. The protein is glycosylated (Mr approximately 27,000) in the presence of microsomal membranes. Northern analysis reveals a 1.2-kb transcript. Treatment with cycloheximide was associated with superinduction of this transcript suggesting that it is an immediate-early gene. The abundance of IEX-1 mRNA increased rapidly after exposure of the cells to ionizing radiation (2-10 Gy), reaching a maximum by 15 min and returning subsequently to basal levels by 4 h. Expression of IEX-1 was also induced significantly by the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA), the protein phosphatase inhibitor okadaic acid, and tumor necrosis factor-alpha, whereas treatment of cells with UV light and H2O2 had little effect on IEX-1 expression. Cells depleted of PKC by prolonged incubation with TPA showed no attenuated IEX-1 response to tumor necrosis factor-alpha. This is the first report of IEX-1, a radiation-inducible glycosylated human protein, whose expression can be mediated through multisignal transduction pathways.