Expansion of CTG repeat within the 3'-untranstated region of the DMPK gene causes the most common neuromuscular disorder, myotonic dystrophy type I (DM1), through a RNA trans-dominant mechanism. Here, we explore Caenorhabditis elegans as a model system to investigate the repeat size-dependent toxic effect by expression of green fluorescent protein (GFP) transcripts with various lengths of untranslatable CUG repeats (CUG5, CUG30, CUG83, CUG125, and CUG213) in body wall muscles. CUG213 animals died during embryogenesis or showed retarded growth at larval stages due to defective muscle development. CUG 125 animals, although can produce offspring, exhibited uncoordinated muscle function, deviated electropharyngeogram, and an age-dependent abnormality in muscle structure. Most CUG83 animals had normal muscle structure and function as those expressing 30 and shorter repeats. Our results demonstrate for the first time that the in vivo toxicity of CUG repeats is repeat length- and growth-regulated and suggest that expanded CUG repeats are sufficient to cause congenital-like phenotypes in living organisms. (c) 2006 Elsevier Inc. All rights reserved.