iPSC colonies were dissociated into a single-cell suspension using Accutase (Thermo Fisher, A11105-01) and resuspended in E8 medium containing 10 μM Rock inhibitor. Cells were counted using an automated cell counter (Chemometec NC-200) and a cell suspension containing an equal amount of each iPSC line was prepared in E8 medium containing 10 μM Rock inhibitor and seeded at 2 × 105 cells per cm2 on 1% Geltrex- (Thermo Fisher, A1413202) coated plates. Each pool of lines contained between 7 to 24 donors. 24 h after plating, neuronal differentiation of the pooled lines to a midbrain lineage was performed as described by13 (link) with minor modifications: 1. SHH C25II was replaced by 100 nM SAG (Tocris, 6390) in the neuronal induction phase. 2. On day 20, the cells were passaged with Accutase containing 20 units/ml of papain (Worthington, LK00031765) and plated at 3.5 × 105 cells per cm2 on 1% Geltrex-coated plates for final maturation. A link to the step-by-step protocol can be found in the URL section.