These enzymes are collectively known as group IV decarboxylases [<cite idref="PUB00001452"/>].Pyridoxal-dependent decarboxylases acting on ornithine, lysine, arginine andrelated substrates can be classified into two different families on the basisof sequence similarities [<cite idref="PUB00003632"/>, <cite idref="PUB00001452"/>].Members of this family while most probably evolutionary related, do not shareextensive regions of sequence similarities. The proteins contain a conserved lysineresidue which is known, in mouse ODC [<cite idref="PUB00002726"/>], to be the site of attachment of thepyridoxal-phosphate group. The proteins also contain a stretch of threeconsecutive glycine residues and has been proposed to be part of a substrate-binding region [<cite idref="PUB00002116"/>].<p>The ornithine decarboxylases catalyse the transformation of ornithine into putrescine. Phylogenetic analysis of the mRNAs from several mammalian species suggests that ODC is encoded by orthologous genes in the different species. Analysis of divergence patterns in a number of subregions showed that the domains have evolved in a noncoordinate fashion. Evolution of each subregion has been episodic, with periods of both rapid and slow divergence, possibly indicating the existence of selection pressures that were exerted in a time- and domain-specific manner during mammalian speciation. The active form of mammalian ODC is a homodimer of 53 kDa subunits (the monomer retains no enzymatic activity). <i>In vitro</i> hybridisation and cross- linkage analysis have suggested that the active site of ODC is formed at the interface of the two monomers via the interaction of the cysteine-360- containing region of one subunit with the lysine-69-containing region of the other [<cite idref="PUB00002726"/>].</p>