BLM, a helicase protein from the RecQ subfamily, participates in DNA replication and repair with a magnesium-dependent ATP-dependent DNA-helicase activity that unwinds single- and double-stranded DNA in a 3'-5' direction. BLM is part of the BRCA1-associated genome surveillance complex (BASC), which contains BRCA1, MSH2, MSH6, MLH1, ATM, BLM, PMS2 and the RAD50-MRE11-NBS1 protein complex. Defects in BLM are the cause of bloom syndrome, characterized by growth deficiency, skin pigmentation disorders, predisposition to malignancy and chromosomal instability.