Brassinosteroids (BRs) are a group of major phytohormones playing critical roles in plant growth and development. Within the last two decades, key events of BR biosynthesis and signal transduction have been gradually elucidated. The detailed molecular mechanisms controlling bioactive levels of BRs, however, are not fully understood. TCP1 is a member of class II TCP proteins in Arabidopsis thaliana. The role of TCP1 in BR biosynthesis was discovered by an activation tagging analysis aiming to screen for genetic suppressors of an intermediate allele named bri1-5 of the BR receptor gene BRI1. Overexpression of TCP1 partially suppresses the defective phenotypes of bri1-5 via direct up-regulation of DWF4, one of the target genes of TCP1.