LRRK2 and Parkinson disease: Most in vitro studies have concluded that the various toxicities caused by overexpression of LRRK2 are dependent on LRRK2 kinase activity.41–43 Though there is limited evidence that LRRK2 itself is overexpressed in PD, most cells in culture appear to have little or no LRRK2 protein that can be detected,44 so overexpressing the protein is required, in many cases, to study LRRK2 function.