PAK4 and Parkinson disease: In contrast to humans and rodents, in which PAK4 activity in DA neurons is reduced under PD disease conditions, which in turn induce DA cell loss (Danzer et al., 2007; Won et al., 2016), we were able to show, for the first time, that mutations in the PAK4 homolog mbt trigger a variety of PD-related phenotypes in Drosophila. Mbt is required in DA PAM neurons for normal life expectancy and climbing performance.