The Arabidopsis INNER NO OUTER (INO) gene is essential for formation and asymmetric growth of the ovule outer integument. INO encodes a member of the newly described YABBY family of putative transcription factors that contain apparent Cys(2)-Cys(2) zinc-finger domains and regions of similarity to the high mobility group (HMG) transcription factors. In wild-type plants, INO is expressed specifically on one side of the central region of each ovule primordium in the cells that give rise to the outer integument. Alterations in the INO expression pattern in mutant backgrounds implicate INO as a positive regulator of its own expression, and ANT, HLL, BEL1, and SUP as direct or indirect negative regulators that help to establish the spatial pattern of INO expression. We hypothesize that INO is necessary for polarity determination in the central part of the ovule. Maintenance of polarity in other parts of ino ovules indicates the existence of additional regulators and provides further evidence that the ovule is a compound structure.