In some cases, such as in S. solfataricus, the HD-type phosphohydrolase/nuclease and the DExD/H-box helicase domains are encoded as separate genes (Han et al, 2009).The HD-type phosphohydrolase/nuclease domain is found in a superfamily of enzymes with either a predicted or known phosphohydrolase activity (Aravind and Koonin, 1998).