These results demonstrate specific associations of BLM and BRCA1 at SB-649868 telomeres in ALT cells. Since APB PLX7904 formation is thought to be integral to telomere maintenance in ALT cells, we assessed whether BLM and BRCA1 co-localize with PML at telomeres. While BLM commonly colocalizes with PML at telomeres, BRCA1 does not. As BRCA1 localization is dependent upon initiation of recombination events including DNA strand breaks, the absence of significant colocalization with PML at telomeres may indicate a transient association or exclusion of BRCA1 association from APBs during telomere metabolism. Association of BLM and BRCA1 was confirmed by co-immunoprecipitation of these proteins from total cell extracts made from ALT-positive U2OS cells at different stages of the cell cycle. Our results show that BRCA1 co-immunoprecipitation with BLM is more pronounced at G2 compared to earlier stages. Extracts prepared from BLM-deficient cells served as a negative control. These data corroborate our immunofluorescence studies. The MRN complex plays a central role in recruitment of BRCA1 to DNA double strand breaks, followed by the recruitment of other recombination proteins to initiate end-resection and strand exchange during recombination. MRN also promotes recruitment of BLM to DSBs to participate in early recombination events. Our data show that BLM and BRCA1 physically interact and the BLM-BRCA1-telomere co-localized foci are specific to ALT cells. In order to discern whether BLM is present during early recombination events in ALT cells, U2OS cell extracts were prepared at different stages of the cell cycle and proteins immunoprecipitated using anti-RAD50. As expected, BRCA1 and RAD50 co-immunoprecipitate from these extracts predominantly in S-phase. BLM also co-immunoprecipitates with RAD50 with an interaction that is more pronounced in G2phase. In order to confirm these interactions in the context of telomeres, U2OS cells were synchronized as before and assessed for co-localization of BRCA1 or BLM with RAD50 at telomeres using immunofluorescence and telomere FISH with a telomeric probe.