In humans defects in three of at least five functionally non-redundant genes encoding RecQ helicases are associated with severe heritable disease, i.e. Bloom's syndrome (BLM, RECQ2, RECQL3 gene), Rothmund-Thomson syndrome (RECQL4 gene) and Werner's syndrome (WRN, RECQ3, RECQL2 gene) (Bennett and Keck, 2004). Here, BLM is linked to Rothmund-Thomson syndrome.