Updated project metadata. Mutations in intrinsically disordered regions (IDRs) of proteins are associated with a wide spectrum of diseases. Since IDRs lack a fixed three-dimensional structure, the molecular mechanism by which such mutations cause disease is often unknown. Here, we employ a peptide-based proteomic screen to investigate the impact of mutations in IDRs on protein-protein interactions. We find that mutations in disordered cytosolic regions of three transmembrane proteins (GLUT1, ITPR1 and CACNA1H) lead to an increased binding of clathrin. In all three cases, the mutation creates a dileucine motif known to mediate clathrin-dependent trafficking. Follow-up experiments on full length GLUT1 (SLC2A1), the glucose transporter causative of GLUT1 deficiency syndrome, revealed that the mutated protein mislocalizes to intracellular compartments in a model cell line and in patient-derived induced pluripotent stem cells. Mutant GLUT1 interacts with adaptor proteins (APs) in vitro, and knocking-down AP-2 reverts the cellular mislocalization. A systematic analysis of other known disease-causing variants revealed a significant and specific overrepresentation of gained dileucine motifs in structurally disordered cytosolic domains of transmembrane proteins. Thus, several mutations in disordered regions appear to cause “dileucineopathies”.