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Reverse transcription-polymerase chain reaction (RT-PCR) with interleukin-1alpha (IL-1alpha)-specific primers using total RNA from lipopolysaccharide (LPS)-stimulated lung macrophages resulted in the amplification of two distinct cDNA fragments. Cloning and sequencing of the canine and feline fragments revealed that, except for the absence of a specific 174 nucleotide sequence, the short and the long transcripts were identical. The in-frame 174 nucleotide deletion corresponds to exon 5 of the human and murine IL-1alpha gene, which encodes the cleavage site for calpain, a protein necessary for the processing of the IL-1alpha precursor into mature IL-1alpha. The two transcripts were found in the dog, cat and pig; analysis by RT-PCR, Southern and Northern blot hybridization showed no expression of the shorter IL-1alpha mRNA in equine or bovine macrophages. Expression of the two canine IL-1alpha transcripts was also detected in synovial membranes and was coordinately up-regulated in response to Borrelia burgdorferi infection under both in-vitro and in-vivo conditions.