Sequence alignment of several ERs including the amphioxus ER and the lamprey ER, as well as ancestral steroid receptors. The DBD is highlighted with light grey. The 12 helices from the LBD are indicated, based on the known 3D structure of human ERα . Amino acids from human ERα making direct hydrogen bonds with E2 are indicated in green. Amino acids making hydrophobic bonds with E2 are highlighted in purple. Amino acids known to be involved in co-activator interaction have been indicated with a star on top of each site . The more divergent A/B domain as well as the F domain have been omitted from the alignment. However, the numbering of the sites along the alignment starts at the beginning of each protein. The exon-intron limits of amphiER and humanERα have been indicated with small red strokes. The sequences of AncSRa and AncSRb have been inferred in this study. The sequence of AncSR1 was retrieved from a previous analysis .