Intron phase (i.e., the position of an intron within a codon; phase 0 when lying before the first base, phase 1 when lying after the first base and phase 2 when lying after the second base) was also assessed for all PtMPK and PtMKK gene models (Figures 3 and 4), as well as for all AtMPK and AtMKK gene models (Figures 5 and 6). For both PtMPKs (58%) and PtMKKs (73%), the majority of introns are within phase 0, while 23% of introns found in both poplar protein kinase families are within phase 2. Phase 1 introns represent 19% of all PtMPKs introns and only 4% of all PtMKKs introns. For Arabidopsis, similar numbers are found (Figures 5 and 6), except that there are no phase 1 introns predicted within AtMKK genes.