Premature termination codons trigger a process in eukaryotes known as nonsense mediated decay or mRNA surveillance, resulting in the rapid decay of the aberrant transcript. Studies in C.elegans have shown this system is mediated by seven smg genes and can prevent the accumulation of toxic, truncated peptides. Here we report the cloning of smg-4 by physical mapping and functional rescue assays. The minimal rescuing activity is found within a genomic operon, encoding a novel protein. The final exon of the gene is alternatively spliced for expression of two different isoforms. Although no known genes were found to exhibit significant homology to smg-4, a novel conserved domain has been identified by alignment with sequences defined by expressed sequence tags (ESTs) from a variety of organisms. Furthermore, we describe a homolog from C.briggsae, which will rescue C.eleganssmg-4 mutants. The C.elegans gene has been fused to green fluorescent protein (GFP). This SMG-4:GFP fusion exhibits nuclear accumulation and diffuse cytoplasmic staining, and further localizes to what appear to be perinuclear and cytoplasmic punctate structures.