Abstract Breast cancer tumor suppressors BRCA1, BRCA2 and PALB2 form a complex that promotes faithful DNA repair by homologous recombination (HR). PALB2, the partner and localizer of BRCA2, mediates the physical and functional communication between BRCA1 and BRCA2. PALB2 binds to BRCA1 through its N-terminal coiled-coiled domain, and to the extreme N-terminal region of BRCA2 via its C-terminal WD40 repeats. Recently, it was shown that BRCA2 and PALB2, like BRCA1, play key roles in the maintenance of the G2/M checkpoint. Here, we show that PALB2 also plays a role in the activation of the G2/M checkpoint. Cells in the mitotic phase were scored on the basis of Ser 10 phosphorylation in the histone, H3 before and one hour after ionizing radiation. PALB2-deficient Fanconi anemia (FA-N) cells exhibited a significant defect in G2/M checkpoint activation, which was corrected upon re-expression of wild-type PALB2. This function of the protein appears to be dependent on its binding to both BRCA1 and BRCA2, as PALB2 mutants abrogated for their BRCA1- or BRCA2-binding abilities were unable to correct the checkpoint activation failure. Since TP53 is often mutated in BRCA- and PALB2-associated breast cancers and since FA-N cells express the SV40 T antigen which inactivates TP53, we studied whether TP53 is involved in PALB2-dependent checkpoint activation. However, depletion of PALB2 in isogenic TP53-wildtype and TP53-null HCT-116 colon cancer cells showed similar defects in checkpoint activation. Furthermore, mouse embryonic fibroblasts (MEFs) derived from a Palb2 knock-in strain, in which the endogenous PALB2-BRCA1 interaction is abrogated, also exhibited a defect in G2/M checkpoint activation. In conclusion, our results demonstrate that PALB2 plays an important role not only in the maintenance but also the activation of the G2/M checkpoint, and suggest that BRCA1, PALB2 and BRCA2 function together as a complex in the maintenance of genome integrity. Citation Format: Srilatha R. Simhadri, Hong Cai, Bing Xia. Role of PALB2 and the BRCA complex in G2/M checkpoint control. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 636. doi:10.1158/1538-7445.AM2013-636