HTRA2 and Parkinson disease: Traditionally, PD has been associated with at least 13 loci and 9 genes, including autosomal dominant forms such as Parkinson disease 1 (PARK1) and SNCA/PARK4, ubiquitin C-terminal hydrolase L1 (UCHL1/PARK5; PARK8/LRRK2; GRB10 interacting GYF protein 2 (GIGYF2/PARK11), and HtrA serine peptidase 2 (HTRA2/PARK13/OMI), as well as autosomal recessive forms like PRKN/PARK2/Parkin; PARK6/PINK1; and PARK7/DJ-1/PARK9/ATP13A2 [12].