Résumé : Recently we have purified to homogeneity and characterized an interferon-induced human protein (p78 protein) which is the equivalent of the interferon-induced murine Mx protein responsible for a specific antiviral state against influenza virus infection. A cDNA library was constructed using mRNAs from interferon-induced human diploid fibroblasts. cDNA clones coding for the human p78 protein were identified and used to determine the chromosomal location of the corresponding gene (termed IFI-78K gene) by hybridization to DNA from a panel of human x rodent somatic cell hybrids. The newly identified gene is located on chromosome 21. This has been confirmed by the observation of a gene dosage effect using chromosome 21 trisomic cells (fibroblasts derived from Down's syndrome patients). Among all interferon-inducible genes mapped so far, the IFI-78K gene is the only one located on chromosome 21, together with the gene for the receptor of type I interferon. Our results also provide further evidence for homology between human chromosome 21 and mouse chromosome 16, since the gene encoding the mouse Mx protein (the presumed mouse homolog protein of human p78 protein) has been assigned to chromosome 16.