Probable transcription factor involved in embryonic patterning. Required for apical embryo development after fertilization. Its specific localization to the apical daughter cell of the zygote, while WOX8 is confined to the basal cell, suggests that the asymmetric division of the plant zygote separates determinants of apical and basal cell fates. Detected in the egg cell and the central cell of the embryo sac, but not in the synergids, the antipodals or the male gametophyte. After fertilization, it is expressed in the zygote. After the first division of the zygote, it is detected exclusively in the apical daughter cell, while WOX8 is expressed in the basal daughter cell. Subsequently, it is expressed in all cells of the 4-cell embryo proper and predominantly in the apical domain of the 8-cell embryo. However, in some 8-cell embryos it is also weakly expressed in the central domain suggesting that expression shifts to the most apical cells during this stage. Expression remains restricted to the apical domain in the 16-cell embryo and the early-globular stage. Not expressed in the apical domain thereafter. However, in heart stage embryos it is weakly expressed in a ring of epidermal cells approximately at the junction of hypocotyl and root. Not expressed in mature embryos, endosperm or postembryonically in inflorescences. Belongs to the WUS homeobox family.