In 1979, a growth-promoting hormone was isolated from rape pollen (Grove et al. 1979). Its crystal structure revealed a hydroxyprolinated steroid featuring a B-ring lactone with structural similarities to the insect molting hormone ecdysone (Fig. 9.1a, b). It was named brassinolide and since its discovery dozens of related brassinosteroids, which can promote cell elongation, division, and differentiation, have been reported from different plant species (Bajguz 2007). Using synthetic brassinolide, brassinosteroid-deficient and -insensitive mutants were identified in forward genetic screens in the model plant Arabidopsis. In this way, the core biosynthetic pathway for the hormone and many components of the brassinosteroid signal transduction cascade were defined. One of the first genes to be cloned was BRI1 (Clouse et al. 1996; Li and Chory 1997), a plasma-membrane-localized recep- tor kinase with an extracellular leucine-rich repeat (LRR) domain, a single membrane spanning helix and a cytoplasmic kinase domain (Li and Chory 1997) (Fig. 9.2). BRI1 is one of ~200 LRR receptor kinases found in the Arabidopsis genome (Shiu and Bleecker 2001). BRI1 acts as receptor for brassinosteroids and binds the steroid hormone using its LRR-domain (Wang et al. 2001; Kinoshita et al. 2005).