The muscleblind-like (MBNL) proteins are tissue-specific alternative splicing regulators. Dysfunction of MBNL has been implicated in the pathogenesis of expanded CUG repeats-associated myotonic dystrophy (DM). In this study, we describe the identification and functional characterization of a Caenorhabditis elegans muscleblind (CeMbl) gene. CeMbl is a single gene alternatively spliced to generate two isoforms (CeMBL-A and CeMBL-B). It displays a high homology with human NIBNL1 in the C3H1 zinc finger domains. CeMbl transcripts are detected in larval and adult stages. However, inactivation of CeMbl by RNA-mediated interference results in muscle phenotype only at adulthood. Immunofluorescence staining using anti-vinculin antibody reveals that the organization of dense body is disrupted in affected worms. Our results demonstrate a growth-dependent requirement of CeMbl on muscle structure and function. They also provide a possible molecular basis for the developmentally regulated toxicity of expanded CUG repeats. (c) 2007 Elsevier Inc. All rights reserved.