It is widely accepted that RET undergoes dimerization and transphosphorylation following Glial cell line-derived neurotrophic factor (GDNF) binding to the GDNF-family receptor:RET complex. Transphosphorylation of specific tyrosine residues is a prerequisite for activation of RET tyrosine kinase activity and downstream signaling (Santoro et al. 1995, Airaksinen et al. 1999, Takeda et al. 2001, Leppänen et al. 2004). However, self-association of RET in the absence of GDNF has been reported and may contribute to the mechanism of RET activation (Kjaer et al. 2006). The stoichiometry and kinetics of ligand-receptor complex formation are not well understood. It is assumed that all GDNF family of ligands (GFL) members interact with their cognate co-receptor and activate RET in a similar manner to GDNF (Airaksinen & Saarma 2002).