iNucl. Acids Res. (10 November 2008), gkn873./ibr /br /Plant hormones are small organic molecules that influence almost every aspect of plant growth and development. Genetic and molecular studies have revealed a large number of genes that are involved in responses to numerous plant hormones, including auxin, gibberellin, cytokinin, abscisic acid, ethylene, jasmonic acid, salicylic acid, and brassinosteroid. Here, we develop an Arabidopsis hormone database, which aims to provide a systematic and comprehensive view of genes participating in plant hormonal regulation, as well as morphological phenotypes controlled by plant hormones. Based on data from mutant studies, transgenic analysis and gene ontology (GO) annotation, we have identified a total of 1026 genes in the Arabidopsis genome that participate in plant hormone functions. Meanwhile, a phenotype ontology is developed to precisely describe myriad hormone-regulated morphological processes with standardized vocabularies. A web interface (http://ahd.cbi.pku.edu.cn) would allow users to quickly get access to information about these hormone-related genes, including sequences, functional category, mutant information, phenotypic description, microarray data and linked publications. Several applications of this database in studying plant hormonal regulation and hormone cross-talk will be presented and discussed. 10.1093/nar/gkn873br /iZhi-Yu Peng, Xin Zhou, Linchuan Li, Xiangchun Yu, Hongjiang Li, Zhiqiang Jiang, Guangyu Cao, Mingyi Bai, Xingchun Wang, Caifu Jiang, Haibin Lu, Xianhui Hou, Lijia Qu, Zhiyong Wang, Jianru Zuo, Xiangdong Fu, Zhen Su, Songgang Li, Hongwei Guo/i