Parkinson’s disease (PD) has been attributed to a combination of genetic and nongenetic factors. We studied a set of monozygotic twins harboring the heterozygous glucocerebrosidase mutation (GBA N370S) but clinically discordant for PD. We applied induced pluripotent stem cell (iPSC) technology for PD disease modeling using the twins’ fibroblasts to evaluate and dissect the genetic and nongenetic contributions. Utilizing fluorescence-activated cell sorting, we obtained a homogenous population of “footprint-free” iPSC-derived midbrain dopaminergic (mDA) neurons. The mDA neurons from both twins had ∼50% GBA enzymatic activity, ∼3-fold elevated α-synuclein protein levels, and a reduced capacity to synthesize and release dopamine. Interestingly, the affected twin’s neurons showed an even lower dopamine level, increased monoamine oxidase B (MAO-B) expression, and impaired intrinsic network activity. Overexpression of wild-type GBA and treatment with MAO-B inhibitors normalized α-synuclein and dopamine levels, suggesting a combination therapy for the affected twin. Display omitted •GBA N370S results in elevated α-synuclein, and high MAO-B increases DA metabolism•iPSC-derived mDA neurons reveal differences between MZ twins discordant for PD•Neuronal purification offers a relatively pure population of mDA neurons•Footprint-free iPSCs, high-throughput MEA, and NanoString are utilized Woodard et al. studied a set of identical twins harboring GBA N370S mutation but clinically discordant for Parkinson's disease (PD). Using iPSC technology, they found that enzymes GBA and MAO-B could be therapeutic targets in PD. These findings shed light into future studies using iPSCs to model idiopathic PD cases.