NGF and Parkinson disease: Further investigation of NGF-KO animals is crucial, especially considering NGF’s implication in multiple neurodegenerative injuries and diseases, such as traumatic brain injury, ischemia, and PD (Goss et al., 1998; Lee et al., 1998; Micera et al., 1998; Brown et al., 2004; Nakagawa and Schwartz, 2004).