On p. 2139, Parker et al. report the identification of a novel gene from two vascular-patterning Arabidopsis mutants, cov1-1 and cov1-2. In the Arabidopsis stem, vascular bundle patterning is highly ordered,consisting of five to eight bundles of xylem and phloem that are separated by interfascicular (IF) tissue. In cov1 mutants, this pattern breaks down in the stem, to produce plants with a continuous ring-like pattern of undefined bundles interspersed with little IF tissue. However, as the authors show, this defect is not caused by altered auxin signalling – an inducer of vascular development. They predict that COV1 encodes an integral membrane protein, which might function in the negative regulation of xylem and phloem differentiation in stems. Future studies should provide new insights into its function and into this important, but little understood, patterning process in plants.