The identification of monogenic variants of Parkinson's disease (PD) has 
provided novel insights into its unknown pathogenesis. As the first protein 
linked to autosomal-recessive forms of PD, Parkin became a welcome tool to 
explain biochemical and neuropathological observations that had suggested 
involvement of the ubiquitin-proteasome system (UPS) in PD. Based on cellular 
expression studies and biochemical in vitro experiments, several researchers 
ascribed an E3-type, E2-dependent ubiquitin protein ligase activity to wild-type 
(but not mutant) Parkin proteins. Although the individual components of the 
proposed Parkin ubiquitin ligase complex in the normal human brain remain to be 
identified and the E3 ligase effect of Parkin function has not yet been 
confirmed in an animal model, the scientific exploration of a protein with 
several links to the UPS has provided many leads in PD research. This chapter 
describes assays that the authors have used to examine the cellular and in vitro 
effects of neural Parkin.