Aromatic and heteroaromatic amines (ArNH2) are activated by cytochrome P450 monooxygenases, primarily CYP1A2, into reactive N-arylhydroxylamines that can lead to covalent adducts with DNA nucleobases. Hereby, we give hands-on mechanism-based guidelines to design mutagenicity-free ArNH2. The mechanism of N-hydroxylation of ArNH2 by CYP1A2 is investigated by density functional theory (DFT) calculations. Two putative pathways are considered, the radicaloid route that goes via the classical ferryl-oxo oxidant and an alternative anionic pathway through Fenton-like oxidation by ferriheme-bound H2O2. Results suggest that bioactivation of ArNH2 follows the anionic pathway. We demonstrate that H-bonding and/or geometric fit of ArNH2 to CYP1A2 as well as feasibility of both proton abstraction by the ferriheme-peroxo base and heterolytic cleavage of arylhydroxylamines render molecules mutagenic. Mutagenicity of ArNH2 can be removed by structural alterations that disrupt geometric and/or electrostatic fit to CYP1A2, decrease the acidity of the NH2 group, destabilize arylnitrenium ions, or disrupt their pre-covalent transition states with guanine.