Other members of the group B and C MPK genes also display notable differences in their expression profiles. The poplar group C PtMPK7 is generally much more highly expressed than its paralog, PtMPK14, in all tested organs except cell cultures. As well, expression of the group B PtMPK5-2 is generally higher than that of PtMPK5-1. The physiological impact of these differences in levels of transcript accumulation among paralogs remains to be resolved. Such a pattern could however be an indicator of pseudogenization of one member of a recently duplicated gene pair, even though signal transduction genes are often thought to be subject to strong conservation constraints [61].