In a screen for morphological mutants from the T1 generation of approximately 50 000 activation-tagging lines, we isolated four dominant mutants that showed hyponastic leaves, downward-pointing flowers and decreased apical dominance. We designated them isoginchaku (iso). The iso-1D and iso-2D are allelic mutants caused by activation of the AS2 gene. The T-DNAs were inserted in the 3' downstream region of AS2. Iso-3D and iso-4D are the other allelic mutants caused by activation of the ASL1/LBD36 gene. These two genes belong to the AS2 family that is composed of 42 genes in Arabidopsis. The only recessive mutation isolated from this gene family was of AS2, which resulted in a leaf morphology mutant. Applying reverse genetics using a database of activation-tagged T-DNA flanking sequences, we found a dominant mutant that we designated peacock1-D (pck1-D) in which the ASL5/LBD12 gene was activated by a T-DNA. The pck1-D mutants have lost apical dominance, have epinastic leaves and are sterile. These results strongly suggest that activation tagging is a powerful mutant-mining tool especially for genes that make up a gene family.