UBQLN2 and amyotrophic lateral sclerosis: Identified ALS-linked mutations in ubiquilin-2 all localize to a proline-rich (PXX repeat) region that is distinct from either the N-terminal UBL (ubiquitin-like) domain that interacts with the proteasome or the C-terminal UBA (ubiquitin-associated) domain that associates with ubiquitinated proteins, suggesting that ALS mutants may leave these functional domains intact.