Principal component analysis (PCA) plot of sensitizers and non-sensitizers. The PCA plot shows the 26 most significant genes, based on p-value from an ANOVA comparing sensitizers with non-sensitizers. Triplicate samples of 18 sensitizers and 20 non-sensitizers are shown. The coloring shows the different chemical reactivity groups included. The sensitizers are centered in the top of the plot and the non-sensitizers in the bottom of the plot. Add/El: Addition/elimination to carboxylic acid derivatives; Koor: Coordination bonds; MA: Michael addition or 1,4-Addition to a/b unsaturated carbonyl; NAR: nucleophilic aromatic substitution, PreH: Prehapten; Chemical activation (outside the skin); ProH: Prohapten; Enzymatic activation; SB: Schiff’s Base reaction; SN2: bimolecular nucleophilic substitution.