STYX, also known as serine/threonine/tyrosine-interacting protein, belongs to the protein-tyrosine phosphatase family. It contains a Gly residue instead of a conserved Cys residue in the dsPTPase catalytic loop which renders it catalytically inactive as a phosphatase. The binding pocket is however sufficiently preserved to bind phosphorylated substrates, and maybe protect them from phosphatases. It seems to play a role in spermiogenesis.