The EXS domain was named after the yeast ERD1, the human XPR1, and the yeast SYG1. In contrast to the SYG1 and XPR1 proteins, which have both a SPX and EXS domain, the majority of proteins in yeast having one of these domains do not harbor the other. In Arabidopsis, only the members of the PHO1 proteins family possess both a SPX and EXS domain. The function of the EXS domain is unknown, although it could be involved in either phosphate transport or sensing or in sorting proteins to endomembrane [1].

Some proteins known to contain a EXS domain are listed below:

• Vertebrate xenotropic and polytropic retrovirus receptor 1 receptor (XPR1), initially identified as a retrovirus receptor but for which the function in uninfected cells remains unknown.
• Yeast SYG1, a protein involved in the mating pheromone signal transduction pathway.
• Yeast ERD1, involved in the localization of endogenous endoplasmic reticulum proteins.
• Arabidopsis thaliana PHO1, a protein involved in the loading of inorganic phosphate into the xylem of roots.
• Arabidopsis thaliana PHO1-like proteins (identified as PHO1;H1 to PHO1;H10).

The profile we developed covers the entire EXS domain.