## Abstract

We have determined the levels of mRNAs for IFN-<latex>$\beta$</latex>, IFN-<latex>$\gamma$</latex> and various <latex>$\alpha$</latex>-IFNs (IFN-<latex>$\alpha$</latex>1, -<latex>$\alpha$</latex>2, -<latex>$\alpha$</latex>4, -<latex>$\alpha$</latex>5, -<latex>$\alpha$</latex>6, -<latex>$\alpha$</latex>7, -<latex>$\alpha$</latex>8 and -<latex>$\alpha$</latex>14) in normal and leukaemic human blood leucocytes and several cell lines induced in different fashions. The ratio of <latex>$\alpha$</latex> to <latex>$\beta$</latex> IFN transcripts varied greatly, depending on the cell type. The levels of the individual IFN-<latex>$\alpha$</latex> RNAs were very different: IFN-<latex>$\alpha$</latex>1, -<latex>$\alpha$</latex>2 and -<latex>$\alpha$</latex>4 RNAs constituted the major fraction of the IFN-<latex>$\alpha$</latex> transcripts measured, while IFN-<latex>$\alpha$</latex>6, -<latex>$\alpha$</latex>7, -<latex>$\alpha$</latex>8 and -<latex>$\alpha$</latex>14 were minor components in normal, induced leucocytes. Moreover, there was a striking difference in the proportion of individual IFN-<latex>$\alpha$</latex> mRNA species in different cell types, in particular between normal and leukaemic cells; for example all cases of myeloblastic leukaemia examined showed a high expression of IFN-<latex>$\alpha$</latex>14. Use of different induction protocols did not significantly affect the proportion of IFN mRNAs. Analysis of the human IFN-<latex>$\alpha$</latex>1 gene by reversed genetics led to the identification of a segment of 5 flanking sequence between positions 117 and 68 upstream of the cap site which is required for inducibility by virus.