The nuclear enzyme PARP1 plays a central role in sensing DNA damage and facilitating repair. Tumors with BRCA1/2 mutations are highly dependent on PARP1 as an alternative mechanism for DNA repair, and PARP inhibitors generate synthetic lethality in tumors with BRCA mutations, resulting in cell cycle arrest and apoptosis. Zhou et al. recently synthesized an 18 F-labeled PARP1 inhibitor ([18 F] FluorThanatrace) for PET, and demonstrated high specific tracer uptake in a xenograft model of breast cancer [1]. In the current study, we characterize the level of baseline PARP expression and activity across multiple human breast cancer cell lines, including a BRCA1 mutant line. PARP expression and activity, as measured by levels of PAR and PARP1, is correlated with in vitro [18 F] FluorThanatrace binding as well as tracer uptake on PET in a xenograft model of breast cancer. Radiotracer uptake in genetically-engineered mouse fibroblasts indicates [18 F] FluorThanatrace is selective for PARP1 versus other PARP enzymes. This motivates further studies of [18 F] FluorThanatrace as an in vivo measure of PARP1 expression and activity in patients who would benefit from PARP inhibitor therapy.