The DDHD domain is 180 residues long and contains four conserved residues that may form a metal binding site. The domain is named after these four residues. This pattern of conservation of metal binding residues is often seen in phosphoesterase domains. This domain is found in retinal degeneration B proteins, as well as a family of probable phospholipases. It has been shown that this domain is found in a longer C terminal region that binds to PYK2 tyrosine kinase. These proteins have been called N-terminal domain-interacting receptor (Nir1, Nir2 and Nir3). This suggests that this region is involved in functionally important interactions in other members of this family.