Sequence alignment of motif I and motif II in the LBD. Alignment of motif I and motif II in the LBD of TRs shows that two amino acids in motif I (a glycine at position 21 and an aspartic acid at position 29) and three amino acids in motif II (an alanine at position 3, a leucine at position 9 and an isoleucine or leucine at postion 45) are invertebrate TR homologue-specific (all highlighted in red). The conserved residues are highlighted in green. DpTR: water flea Daphnia pulex TR, LgTR: owl limpet Lottia gigantean TR, SmTRα: blood fluke S. mansoni TR alpha, SmTRβ: S. mansoni TR beta, SpTR: sea urchin Strongylocentrotu purpuratus TR. The accession numbers of the aligned human nuclear receptors can be found in Additional files 1 and 2.