A novel transposon-tagging strategy designed to recover dominant gain-of-function alleles was performed with Arabidopsis by using a Dissociation element with a cauliflower mosaic virus 35S promoter transcribing outward over one terminus. Lines containing transposed copies of this transposon were screened for mutants, and a semidominant mutation affecting plant height, hypocotyl elongation, and fertility was recovered. The pleiotropic effects of this mutation appear to result from a general reduction in cell expansion, and some of the effects are similar to those caused by supplying exogenous ethylene or cytokinin to wild-type seedlings. In addition, the arrangement of cells in some organs such as the etiolated hypocotyl, is disorganized. The mutation was called tiny, and the affected gene was cloned by first using transposon sequences to isolate the mutant allele. The predicted protein product of the TINY gene shows strong homology with the DNA binding domain of a recently identified class of plant transcription factors. This domain, called the APETALA2 domain, was initially identified as a duplicated region within the APETALA2 gene of Arabidopsis and then as a conserved region between APETALA2 and the ethylene responsive element binding proteins of tobacco. In the mutant allele, the Dissociation element is inserted in the untranslated leader of the TINY gene, 36 bp from the ATG, and the mutant contains a novel transcript that initiates from the cauliflower mosaic virus 35S promoter within the transposon. This transcript is present in greater abundance than the wild-type TINY transcript; therefore, the semidominant tiny mutation most likely results from increased, or ectopic, expression of the gene.