It is well-known that base sequences in the protein-coding regions of DNA molecules exhibit a period-3 behavior, which is the theoretical basis of gene prediction. In this work, period-3 behavior of the exons was studied by power spectrum, and the results showed that many exons had no period-3 behavior. After these exons in gene were connected to code proteins, the long coding-protein sequences then displayed the period-3 behavior. The period-3 behavior is related to the distribution of bases and the synonymous codons usage. Each exon was adjusted to make it accord with the translation order, and it has been found that the synonymous codon usage preference is another character of exons. These conclusions would be sig- nificant to gene prediction and the study on introns.

Key words: power spectrum, exon, protein-coding region, periodicity