MUL1 and Parkinson disease: Improper recycling and resulting protein aggregation readily occurs in neurons, as they do not proliferate (Rubinsztein, 2006; McDonald, 2021), and has also been implicated in neurodegenerative diseases, including cytotoxicity in amyotrophic lateral sclerosis (Muzio et al., 2020), amyloid-B aggregation in Alzheimer’s disease (Small et al., 2005; Rogaeva et al., 2007; Lane et al., 2010), and MUL1 degradation in Parkinson’s disease (Tang et al., 2015).