Limited functional information is available for group D MKKs, of which there are five encoded representatives in the poplar genome (PtMKK7, PtMKK9, PtMKK10, PtMKK11-1 and PtMKK11-2). However, our RTqPCR analysis suggests that only PtMKK7 and PtMKK9 are clearly expressed (Figure 10). The other three genes may therefore be pseudogenes, or be expressed only under circumstances that were not tested in our survey. The pseudogene hypothesis is also supported by the observation of structural differences in normally conserved motifs within the predicted PtMKK10, 11-1 and 11-2 protein kinase domains, and by the apparent absence of expression data for the Arabidopsis (AtMKK10) and rice (OsMKK10-1/10-2) putative orthologs [39].