A method for reducing the level of nornicotine or N'-nitrosonornicotine in a plant of the genus Nicotiana or plant part thereof, or in a tobacco product prepared from said plant, said method comprising: (i) inhibiting the expression or function of a cytochrome P450 by mutating a gene in said plant by transposon tagging or by random or targeted mutagenesis, wherein said gene encodes a cytochrome P450 polypeptide comprising an amino acid sequence selected from the group consisting of: a) an amino acid sequence set forth in SEQ ID NO: 4 or 6; b) an amino acid sequence that is at least 95% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; c) an amino acid sequence that is at least 98% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and d) an amino acid sequence comprising at least 50 consecutive amino acids of SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and (ii) selecting for a plant that has reduced cytochrome P450 activity. (i) inhibiting the expression or function of a cytochrome P450 by mutating a gene in said plant by transposon tagging or by random or targeted mutagenesis, wherein said gene encodes a cytochrome P450 polypeptide comprising an amino acid sequence selected from the group consisting of: a) an amino acid sequence set forth in SEQ ID NO: 4 or 6; b) an amino acid sequence that is at least 95% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; c) an amino acid sequence that is at least 98% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and d) an amino acid sequence comprising at least 50 consecutive amino acids of SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and a) an amino acid sequence set forth in SEQ ID NO: 4 or 6; b) an amino acid sequence that is at least 95% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; c) an amino acid sequence that is at least 98% identical to an amino acid sequence set forth in SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and d) an amino acid sequence comprising at least 50 consecutive amino acids of SEQ ID NO: 4 or 6, wherein the polypeptide is involved in the metabolic conversion of nicotine to nornicotine in a plant of the genus Nicotiana or a plant part thereof; and (ii) selecting for a plant that has reduced cytochrome P450 activity.