The use of gene expression profiling coupled with reverse genetics offers great potential to rapidly and efficiently identify gene functions. To identify genes involved in secondary cell wall formation in Arabidopsis, Brown et al. (pages 2281–2295) used microarray data generated from developing stems and hypocotyls to identify genes coregulated with the secondary cell wall-specific markers IRREGULAR XYLEM1 (IRX1), IRX3, and IRX5. By cross-referencing publicly available data, 16 genes were selected for reverse genetic analysis. Seven of the T-DNA insertional mutant lines isolated showed an irregular xylem phenotype characteristic of a secondary wall defect. The cover shows a vascular bundle from an inflorescence stem stained with Toluidine blue, altered to highlight the secondary walls in yellow. The section is from an irx6 mutant plant containing an insertion in COBRA-LIKE4. irx6 plants exhibit decreased cellulose incorporation into the secondary wall that results in irregularly shaped and collapsed xylem vessels.