It is advantageous for an organism to be able to remove aberrant mRNAs that have either been incorrectly transcribed or processed in order to prevent the accumulation of potentially harmful proteins. The selective degradation of nonsense containing transcripts has been described in yeast. Caenorhabditis elegans and plants. The ease of identification of new mutant alleles in the AUX1 gene of Arabidopsis thaliana has provided a useful system to study novel mutations affecting mRNA stability and pre-mRNA splicing. To date 50 alleles of AUX1 have been identified of which 14 have been characterised at the sequence level. Eight of the characterised alleles encode missense mutations while the others cause nonsense mutations or splicing defects. The 2 splicing mutants identified affect the 5' or 3' splice sites and lead to cryptic splicing events resulting in premature stop codons. The AUX1 mRNA levels of the nonsense containing mutants are reduced compared to the wild-type or missense mutants whereas those of a control transcript (SecY) are unaltered. This provides further evidence for a nonsense-mediated mRNA degradation mechanism in plants and provides a system to study the phenomenon further in Arabidopsis.