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The mechanism of gene duplication as the means to acquire new genes with previously nonexistent functions is inherently self limiting in that the function possessed by a new protein, in reality, is but a mere variation of the preexisted theme. As the source of a truly unique protein, I suggest an unused open reading frame of the existing coding sequence. Only those coding sequences that started from oligomeric repeats are likely to retain alternative long open reading frames. Analysis of the published base sequence residing in the pOAD2 plasmid of Flavobacterium Sp. K172 indicated that the 392-amino acid-residue-long bacterial enzyme 6-aminohexanoic acid linear oligomer hydrolase involved in degradation of nylon oligomers is specified by an alternative open reading frame of the preexisted coding sequence that originally specified a 472-residue-long arginine-rich protein.
Shigeo Ohno (Sun,) studied this question.