At the level of MAPKKs, phylogenetic conservation of exon length and of exon-intron junctions is also generally observed. Hence, in group A MKKs, both the AtMKK2 and AtMKK6 coding regions are composed of eight exons, which is identical to the structure of their respective predicted orthologs in poplar, PtMKK2-2 and PtMKK6. In addition intron phase configuration is identical among these genes. On the other hand, the AtMKK1 coding region consists of six exons, a pattern that is not conserved in the coding region of the closest poplar putative ortholog, PtMKK2-1, which is constituted of nine exons. For group B MKKs, despite the high level of predicted protein sequence similarity between AtMKK3 and PtMKK3, the number of exons within their respective coding regions differs (eight exons for AtMKK3; nine for PtMKK3). However, this lack of conservation in the exon and intron organization is not caused by the kinase domain evolutionary status. In fact, the regions encoding the protein kinase domain are well conserved in terms of exon count, exon length and intron phase and differences between these two gene models are found at the end of the coding regions (within the NTF2 domain of the corresponding encoded proteins). Finally, as reported earlier [4], both the Arabidopsis group C and D MKKs display an intronless configuration. This trait is fully conserved within both poplar group C and group D MKK gene families.