An extensive computer-assisted analysis of known pre-proinsulin coding sequences has shown correlations that can be interpreted as evidence for an intron-mediated juxtaposition of exons in the evolution of these genes. The evidence includes the discovery that the regions of the pre-proinsulin genes that code for the signal peptide consist of nearly tandem repeating units of nine base pairs. This pattern reappears in the C region of the genes after a large intron that occurs in three of the four genes analyzed. A model is proposed in which primordial insulin was coded for by two separate minigenes arising from a gene duplication, each with identical or nearly identical signal peptide coding regions. The minigenes fused into one transcriptional unit mediated by the large intron, and the signal peptide coding region of one of the putative minigenes evolved into the latter portion of the C peptide coding region.