We had identified brassinosteroid biosynthesis inhibitor Brz insensitive mutant bill from EMS-mutation lines, and bil5 from fast neutron-mutation lines. We tried to screen new bil mutants from Arabidopsis activation-tag lines and selected bil3 and bil4 that showed long hypocotyl even with Brz in the dark, slender dwarf phenotype related to excessive epinastic leaves and thin influorescences in the light. Genetic analysis suggested that the candidate genes of bil4 mutation are a novel protein. Transgenic plants overexpressing the bil4 candidte gene display a slender dwarf phenotype and are insensitive to Brz. Though the phenotypes of bil3 and bil4 are very similar, a possible bil3 gene is a novel protein that is encoded on the different chromosome with bil4. We are now establishing transgenic plants that overexpress the candidate genes to identify relation between the genes and bil3/bil4 mutants. In the comparison these mutants with mutation genes, we would like to clarify in detail the mechanism of brassinosteroid signaling transduction.