Together with norvaline, norleucine is found in small amounts in some bacterial strains where its concentration can approach millimolar.

The incorporation of Nle into peptides reflects the imperfect selectivity of the associated aminoacyl-tRNA synthetase.

In Miller–Urey experiments probing prebiotic synthesis of amino acids, norleucine and especially norvaline are formed.

[6] For this reason, norleucine has been used to probe the role of methionine in Amyloid-β peptide (AβP) the central constituent of senile plaques in Alzheimer's disease.

A study showed that with the substitution of the methionine at the 35 position with norleucine the neurotoxic effects of the Aβ peptides were completely negated.