In higher primates, the coding region of the gene for involucrin, an epidermal protein, is mostly composed of a recently generated (modern) segment of repeats of a sequence of 10 codons. While the rest of the coding region has evolved only by nucleotide substitutions, the modern segment has evolved by successive addition of repeats. This process has not taken place randomly; instead, the expansion of the modern segment has been progressive from 3' to 5' end, thus adding vectorially regions that have been defined as early, middle, and late. The relatedness of the human, chimpanzee, and gorilla may be analyzed with greatest sensitivity by comparing their middle regions. The chimpanzee involucrin gene is more closely related to that of the gorilla than to that of the human.