Why ornithine is unlikely to be found in proteins in vivo mcat?

Ornithine is unlikely to be found in proteins in vivo mcat because not every amino acid found in the human body is identified by a codon in the genetic code or merged into proteins, that is Ornithine.

Ornithine is found as one of the intermediates in the urea cycle, the process of metabolism when the body excretes excess nitrogen. 

what is the statement?

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