Similar to ALS patient fibroblasts, HeLa cells expressing GFP-FUSR521C and GFP-FUSP525L were more sensitive to CPT than HeLa cells expressing wild-type GFP-FUS, supporting the notion that FUS mutations associated with ALS confer hypersensitivity to TOP1-induced DNA breakage. The gene discussed is FUS; the disease is amyotrophic lateral sclerosis.