<p>The haloacid dehydrogenase (HAD) superfamily includes phosphatases, phosphonatases, P-type ATPases, beta-phosphoglucomutases, phosphomannomutases, and dehalogenases, which are involved in a variety of cellular processes ranging from amino acid biosynthesis to detoxification[<cite idref="PUB00003337"/>].</p><p>Crystal structures of proteins from the HAD superfamily show that these proteins all share a conserved alpha/beta-domain classified as a hydrolase fold, which is similar to the Rossmann fold [<cite idref="PUB00026943"/>]. This conserved domain usually contains an insertion (sub)domain. For example, the crystal structure of a phosphoglycolate phosphatase from Thermoplasma acidophilum [<cite idref="PUB00026943"/>] revealed two distinct domains, a larger core domain and a smaller cap domain. The large domain is composed of a centrally located five-stranded parallel beta-sheet with strand order S10, S9, S8, S1, S2 and a small beta-hairpin, strands S3 andS4. This central sheet is flanked by a set of three aplha-helices on one side and two helices on the other. The topology of the large domain is conserved; however, structural variation is observed in the smaller domain among the different functional classes of the haloacid dehalogenase superfamily.</p><p>This entry represents the large domain with conserved topology among the HAD superfamily.</p>