We describe the isolation and characterization of cDNAs for FMO2 from the laboratory rat. In contrast to FMO2 in other animals, each of which contain 535 amino acid residues, analysis of the sequence of the cDNAs and of a section of the corresponding gene revealed that the ORF of the laboratory rat FMO2 encodes a polypeptide of only 432 residues. This truncated protein is due to the presence of a double deletion corresponding to 1263 and 1264 nucleotides of the orthologous FMO2 cDNAs. This double deletion provokes a frame-shift, with the appearance of a premature stop codon in position 1297-1299. By Northern blotting, the probe for FMO2 hybridized a 2.5-kb transcript in lung and kidney samples only. Heterologous expression of the cDNA revealed that the truncated protein was catalytically inactive. By Western blotting, FMO2 was faintly detected at similar to50 kDa in laboratory rat lung. (C) 2002 Elsevier Science (USA).