The mutant human cell line 11, i is unresponsive to interferon a. Here we describe the genetic complementation of this mutant and the identification and cloning of the wild-type gene that corrects the defect. Using transfection with genomic DNA in conjunction with a powerful back-selection, we isolated a cosmid that reverts the mutant phenotype of 11, l cells. The cosmid encodes a single message whose level Is greatly reduced in mutant cells. Complementary DNAs were cloned and found to be virtually identical to tyk2, a human mRNA encoding a non-receptor protein tyrosine kinase of previously unknown function. This finding show8 that fyk2 links the Interferon a/6 receptor to the cytoplasmic transcription factor that mediates activation of interferon-responsive genes.