Lignin is a polymer of monolignols and a major cell wall component in all vascular plants. It plays an important role in water/nutrient transport, imparting cell wall rigidity and impeding microorganism invasion. It has been thought for more than 30 years that the biosynthesis of guaiacyl (G) and syringyl (S) monolignols from coniferaldehyde and sinapaldehyde, respectively, in angiosperms is mediated by a single type of enzyme, cinnamyl alcohol dehydrogenase (CAD). In contrast to this traditional model, on page 1567 of this issue, Li et al. discovered a novel aspen (Populus tremuloides) gene encoding sinapyl alcohol dehydrogenase (SAD) and demonstrated that SAD is sinapaldehyde-specific for syringyl monolignol biosynthesis while CAD is in fact coniferaldehyde-specific for guaiacyl monolignol biosynthesis. The protein immunolocalization and lignin histochemistry verified that, in aspen stem vascular cells, SAD is spatiotemporally linked to S-lignin formation while CAD is associated with G-lignin deposition. The cover shows that CAD protein is immunolocalized in aspen stem secondary xylem elements undergoing strong guaiacyl lignin deposition.