The murine influenza virus resistance locus Mx consists of more than one interferon-responsive transcription unit. We isolated clones of the transcripts of a second Mx gene (Mx2) which is closely linked to the well-characterized resistance gene and determined their sequences. Mx2 mRNA is more than 90% identical to Mx1 mRNA in the region corresponding to the amino termini of the encoded proteins. Mx2 mRNAs of mouse strains BALB/c and CBA have open reading frames apparently interrupted by mutations. Interferon-treated cells from strain A2G and other influenza virus-resistant strains failed to express detectable amounts of Mx2 transcripts.