ATP-dependent DNA helicase that unwinds single- and double-stranded DNA in a 3'-5' direction (PubMed:9840919). Participates in DNA replication and repair (By similarity). Involved in 5'-end resection of DNA during double-strand break (DSB) repair: unwinds DNA and recruits DNA2 which mediates the cleavage of 5'-ssDNA (PubMed:9840919). Negatively regulates sister chromatid exchange (SCE) (PubMed:9840919, PubMed:27010503). Stimulates DNA 4-way junction branch migration and DNA Holliday junction dissolution. Binds single-stranded DNA (ssDNA), forked duplex DNA and DNA Holliday junction (By similarity). Recruited by the KHDC3-OOEP scaffold to DNA replication forks where it is retained by TRIM25 ubiquitination, it thereby promotes the restart of stalled replication forks. ATP + H2O = ADP + H(+) + phosphate Binds 1 zinc ion per subunit. Monomer. Homodimer (via N-terminus). Homotetramer (via N-terminus); dimer of dimers. Homohexamer (via N-terminus). Self-association negatively regulates DNA unwinding amplitude and rate. Oligomer complexes dissociate into monomer in presence of ATP. 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. This association could be a dynamic process changing throughout the cell cycle and within subnuclear domains. Interacts with RMI complex. Interacts directly with RMI1 (via N-terminal region) component of RMI complex. Found in a complex, at least composed of BLM, RAD51 and SPIDR; the complex formation is mediated by SPIDR. Interacts with the KHDC3/FILIA-OOEP/FLOPED scaffold complex and TRIM25 at DNA replication forks (PubMed:29125140). Interacts with ubiquitinated FANCD2 (By similarity). Interacts with SUPV3L1 (By similarity). Interacts with TOP3A (via N-terminal region). Interacts with SPIDR (via C-terminal region); the interaction is direct and required to target BLM to sites of DNA damage. Localized to DNA replication forks, especially after DNA damage. Highly expressed in testis 12-14 days after birth (corresponding to the pachytene phase) and at much lower levels in brain, heart, liver, lung, thymus, kidney and spleen (PubMed:9840919, PubMed:27010503). Expressed in bone marrow (PubMed:27010503). The N-terminal region mediates dimerization and homooligomerization. Both the helicase ATP-binding domain and the helicase C-terminal domain form intramolecular interactions with the HRDC domain in a ATP-dependent manner. The HRDC domain is required for single-stranded DNA (ssDNA) and DNA Holliday junction binding. Poly-ubiquitinated by TRIM25 at Lys-264. Phosphorylated in response to DNA damage. Phosphorylation requires the FANCA-FANCC-FANCE-FANCF-FANCG protein complex, as well as the presence of RMI1 (By similarity). Belongs to the helicase family. RecQ subfamily.