PRKN and Parkinson disease: There are at least three possible reasons preventing us from isolating parkin by immunoprecipitation: (1) We have only limited amounts of postmortem human PD brain to start with, especially from the dopamine fiber-rich regions of the corpus striatum (mainly caudate); (2) The amounts of "Soluble" parkin available for immunoprecipitation are relatively low compared to SDS or urea-dissolved "Insoluble" parkin; and (3) It is often observed that SDS or urea used on insoluble proteins may interfere with the ability of the antibodies to immunoprecipitate.