The present invention relates to a mutant, non-naturally occurring or transgenic tobacco plant cell comprising: (i) a polynucleotide comprising, consisting or consisting essentially of a sequence encoding a functional nicotine N-demethylase and having at least 95% sequence identity to SEQ ID NO:2; (ii) a polypeptide encoded by the polynucleotide set forth in (i); (iii) a polypeptide comprising, consisting or consisting essentially of a sequence encoding a nicotine N-demethylase and having at least 95% sequence identity to SEQ ID NO:3; or (iv) a construct, vector or expression vector comprising the isolated polynucleotide set forth in (i), and wherein the expression or activity of said nicotine demethylase is reduced as compared to a control tobacco plant cell in which the expression or activity of said nicotine demethylase has not been reduced.