the "adra2b" gene (sometimes referenced as "a2ab") is used in animals as a nuclear dna phylogenetic marker. this intronless gene has first been used to explore the phylogeny of the major groups of mammals, and contributed to reveal that placental orders are distributed into four major clades: xenarthra, afrotheria, laurasiatheria, and euarchontoglires. comparative analysis of the primary protein sequence of adra2b across placentals also showed the high conservation of residues thought to be involved in agonist binding and in g protein–coupling. however, great variations are observed in the very long, third intracellular loop, with a polyglutamyl domain displaying pervasive length differences.