The type found in the bacteriophage T7 was recently found to have a hexameric ring structure.  
Defects in this class of enzymes can result in Bloom's syndrome and Werner syndrome.  The type found in the bacteriophage T7 was recently found to have a hexameric ring structure.  Common types in various organisms include UvrD, PhoH, and YjhR, though the first to have its structure determined was PrcA.  Generally requiring a 5-prime overhang for operation, this enzyme uses the energy from the splitting of a phosphate off of dTTP during nucleotide pair hydrolysis.  Preceding DNA polymerase III, FTP, identify this enzyme that unwinds the DNA strand during replication.
Defects in this class of enzymes can result in Bloom's syndrome and Werner syndrome.  
helicase