Although the lower doses tested in the continuous dosing study were not associated with toxicologically relevant findings, and a 4-week brain dose of 840 μg GDNF (Ci: 0.2 μg/μL) was defined as the NOAEL, the cerebellar toxicity with the high dose was of significant concern (in particular because it was not clinically monitorable and did not produce MRI signals) and contributed to a temporary halt of the GDNF clinical development program in PD in 2004 (Luz et al. 2016). The gene discussed is GDNF; the disease is Parkinson disease.