We report here the complete coding sequence of a 203 cDNA, a member of the interferon-inducible Ifi 200 gene family. By combining reverse-transcriptase PCR and rapid amplification of cDNA ends (RACE) techniques we have obtained a 3.8-kb cDNA corresponding to a 203 mRNA. When used as a probe in northern analysis, its 3' segment hybridized to a 3.8-kb interferon-inducible mRNA, whereas the S'-end additionally hybridized to a less abundant interferon-inducible 1 .8-kb mRNA. Nucleotide sequence analysis revealed that the two mRNAs share the 5'-untranslated region and the same open reading frame, which encodes a hydrophilic protein composed of 408 amino acids. The difference between them is due to a 3'-untranslated region extended by alternative polyadenylation site selection. Furthermore, 203 mRNA was found to be inducible by interferon-a in various murine cell lines. Using polyclonal antibodies raised against a segment specific for the 203 protein, we established that p203 protein levels increase on treatment with interferon-a in murine fibroblasts and that p203 is located in the nucleus. MATERIALS AND METHODSCells and culture conditions. Balb/c 3T3, NIH 3T3 and BLKS cells (murine fibroblasts) were grown as monolayers in Dulbecco's modified Eagle's medium (GibcoiBRL) supplemented with 10% calf serum (Gibco/BRL; Balb/c 3T3 and NIH 3T3 cells) or 10% fetal calf serum (Gibco/BRL; BLKS cells). L12R4 (murine T lymphoma cell line) and L1210 (murine pre-B lymphoma cell line) cells were grown in RPMI 1640 me-

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