In the topologic diagram used by Shin et al1 to depict the transmembrane proton-coupled folate receptor, the location of conserved aspartate residues is shown. Mutation of aspartate to tyrosine at position 156 is responsible for instability of the receptor and consequent hereditary folate malabsorption in the patient whom they describe. The aspartate at position 109 is essential for stability of the molecule. (Professional illustration by Alice Chen.)
