Publisher Summary This chapter discusses exons and evolution of proteins. The splitting of most eukaryotic structural genes into coding sequences (exons) and noncoding sequences (introns) is widely interpreted in terms of evolutionary processes concerned with the gene products. There are two distinct aspects of the gene mosaic that assist the production of novel gene products: (1) if exonic regions correspond to individual protein functions, recombination within the intronic regions can reassort these functions into novel protein products and (2) point mutations at the exon/intron splice junctions can result in the deletion or addition of whole blocks of amino acid sequences, while variable splicing can allow transcription of both the original and new gene products simultaneously. The chapter also describes protein structure, correlation of exons with protein structure, and temporal stability of the exon/intron pattern.