Brassinosteroids (BRs) are perceived by a cell surface receptor kinase BRI1, and then various components are predicted to be involved to transduce the BR signal in the cell. To identify additional components involved in BRs signal, we had analyzed various dwarf and semidwarf mutants from Arabidopsis mutant, which is insensitive to the BRs synthetic inhibitor Brz. We had identified 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 bil4 that showed Brz-insensitive phenotype in the dark. 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. From genetic and functional analyses to bil4 mutant, we would like to clarify in detail the mechanism of brassinosteroid signaling transduction.