This family of proteins contain three highly conserved amino acids: one arginine and two aspartates, hence the name of RDD family. This region contains two predicted transmembrane regions. The arginine occurs at the N-terminus of the first helix and the first aspartate occurs in the middle of this helix. The molecular function of this region is unknown. However this region may be involved in transport of an as yet unknown set of ligands (Bateman A pers. obs.).