Positive regulator of brassinosteroid signaling. Homodimer. Expressed in stems and flowers. Induced by flagellin, jasmonic acid (JA), brassinosteroid and cytokinin, and repressed by abscisic acid. Insensitive to gibberellic acid. No visible phenotype. Redundant with BEE1 and BEE3. May be due to a competing acceptor splice site.