AP1S1 and MEDNIK syndrome: Our study is the first to address the pathogenesis of the AP1S1-related enteropathy; rare cell biological studies on MEDNIK syndrome were based on the observation of elevated hepatic copper content in one such patient, and indicated that AP1S1 was essential for translocation of the copper transporter ATP7A to the plasma membrane (Martinelli et al. 2013), whereas copper transporter ATP7B localization was not affected (Overeem et al. 2019).