We have cloned and sequenced a mouse cDNA encoding the 793-residue amino acid sequence of furin, which is a protein homologous to the yeast Kex2 protease. The entire sequence is 94% identical to that of human furin, and it contains the 289-residue sequence of the subtilisin-like catalytic domain. Within this region, 99, 64, and 53% of the amino acids are identical to those of human furin, human PC2 (the other mammalian Kex2-like protein), and yeast Kex2, respectively. It has been proposed that furin is a mammalian prohormone processing enzyme which cleaves precursors at paired basic amino acids, based on the fact that the Kex2 protease is responsible for processing of alpha-mating factor and killer toxin precursors at dibasic sites. However, Northern blot analysis has revealed that a furin mRNA transcript is present in all tested mouse tissues and culture cell lines, including those known not to process prohormones. Moreover, when furin and a prohormone, prorenin, have been coexpressed in mammalian cells by DNA transfection, no processing has been observed. These observations fail to show a role for furin, a Kex2-like mammalian protease, in prohormone processing.