In mouse, at least two catalytically active splice variants (mSMOaand mSMOl) of the flavin-containing spermine oxidase enzyme are present. We have demonstrated previously that the cytosolic mSMOais the major iso-form, while the mSMOlenzyme is present in both nuclear and cytoplasmic compartments and has an extra protein domain corresponding to the addi-tional exon VIa.