Unrooted maximum likelihood tree of Populus (Pt), Prunus (Pp) and Arabidopsis CRE1 - and CKI1 -like two-component histidine kinase genes. CRE1-like genes encode cytokinin receptors. In contrast, CKI1 does not represent a true receptor: despite being able to activate cytokinin phophorelay, it is unable to bind cytokinin. All the other members of Arabidopsis two-component receptor family, which have no known role in cytokinin signaling, are also included. They include ethylene receptors (ETR1, ETR2, ERS1, ERS2 and EIN4), phytochromes (PHYA-E), a putative osmosensor (AtHK1), and a histidine kinase (CKI2/AHK5) associated with ethylene and ABA signaling. The tree is based on a deduced amino acid (113 aa) sequence alignment (Additional file 7: Figure S4). Support for each clade is given as ≥50% of bootstrap pseudoreplicates.