Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease 
affecting motor neurons. Mutations in related RNA-binding proteins TDP-43, 
FUS/TLS and TAF15 have been connected to ALS. These three proteins share several 
features, including the presence of a bioinformatics-predicted prion domain, 
aggregation-prone nature in vitro and in vivo and toxic effects when expressed 
in multiple model systems. Given these commonalities, we hypothesized that a 
related protein, EWSR1 (Ewing sarcoma breakpoint region 1), might also exhibit 
similar properties and therefore could contribute to disease. Here, we report an 
analysis of EWSR1 in multiple functional assays, including mutational screening 
in ALS patients and controls. We identified three missense variants in EWSR1 in 
ALS patients, which were absent in a large number of healthy control 
individuals. We show that disease-specific variants affect EWSR1 localization in 
motor neurons. We also provide multiple independent lines of in vitro and in 
vivo evidence that EWSR1 has similar properties as TDP-43, FUS and TAF15, 
including aggregation-prone behavior in vitro and ability to confer 
neurodegeneration in Drosophila. Postmortem analysis of sporadic ALS cases also 
revealed cytoplasmic mislocalization of EWSR1. Together, our studies highlight a 
potential role for EWSR1 in ALS, provide a collection of functional assays to be 
used to assess roles of additional RNA-binding proteins in disease and support 
an emerging concept that a class of aggregation-prone RNA-binding proteins might 
contribute broadly to ALS and related neurodegenerative diseases.