Involucrin, loricrin and the small proline-rich proteins (SPRRs) are precursors of the cornified envelope of terminally differentiated keratinocytes. The genes for these proteins are closely linked on mouse chromosome 3. Each of the proteins is encoded by a single exon and is largely composed of a segment of short tandem repeats. No size polymorphism of either loricrin or the SPRRs was observed. In contrast, involucrin was found in at least eight polymorphic forms of different size with molecular weights ranging from 51 to 82kDa. Two classes of involucrin alleles were identified. Size polymorphism of involucrin has resulted from the recent expansion of the segment of repeats in one class of alleles, but not in the other. In expanding alleles, repeats were added at a precise location within the segment of repeats, in a 5'-to-3' direction. A study of a large number of allele-specific markers, located on both sides of the site of repeat addition, revealed no evidence for recombination between any of the alleles examined. Expansion of the segment of repeats of the gene for mouse involucrin must result from an intra-allelic process controlled by a cis-acting element, active in one class of alleles, and inactive in the other.