To understand the mechanism by which FUS mutants may contribute to ALS pathogenesis, we determined whether the expression patterns induced by mutant FUS were more similar to reduced FUS expression or overexpressed wild-type FUS. Similarity to reduced expression of FUS would suggest that the mutants act by a loss-of-function mechanism, whereas similarity to overexpressed wild-type FUS would suggest a gain-of-function for the mutants. Here, FUS is linked to amyotrophic lateral sclerosis.