In addition, GDF5 has been shown to promote neurite growth in cultured rat dopaminergic (O'Keeffe et al., 2004a, 2004b; Hegarty et al., 2014) and sympathetic neurons (O’Keeffe et al., 2016), and in the SH-SY5Y human neuroblastoma cell line (Hegarty et al., 2013), which are widely used in vitro models of PD. This evidence concerns the gene GDF5 and Parkinson disease.