Thenotabilis(not) mutant of tomato has a wilty phenotype due to a deficiency in the levels of the plant hormone abscisic acid (ABA). The mutant appears to have a defect in a key control step in ABA biosynthesis – the oxidative cleavage of a 9-cisxanthophyll precursor to form the C15 intermediate, xanthoxin. A maize mutant,viviparous 14(vp14) was recently obtained by transposon mutagenesis. This maize genetic lesion also affects the oxidative cleavage step in ABA synthesis. Degenerate primers for PCR, based on the VP14 predicted amino acid sequence, have been used to provide probes for screening a wilt-related tomato cDNA library. A full-length cDNA clone was identified which is specific to thenotgene locus. The ORFs of the tomato cDNA and maizeVp14are very similar, apart from parts of their N-terminal sequences. Thenotmutation has been characterized at the DNA level. A specific A/T base pair deletion of the coding sequence has resulted in a frameshift mutation, indicating thatnotis a null mutant. This observation is discussed in connection with the relatively mild phenotype exhibited bynotmutant homozygotes.