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One of the hallmarks of cancer cells is their failure to respond to the cellular mechanism of apoptosis. The B-cell lymphoma 2 (BCL-2) family of proteins regulate apoptosis. Their ability to do so can be measured using several methods that in turn anticipate the fate of the cancer cell in response to apoptosis-inducing treatment. These assays ultimately identify the readiness of the cancer cell to undergo apoptosis, which is referred to as the mitochondrial priming state. These metrics, however, have been challenging to implement in the clinic. : Here, we describe a unique method that relies on a panel of novel conformation-specific antibodies (termed PRIMAB) that can directly measure the mitochondrial priming state. These reagents are highly specific for complexes of their corresponding pro-survival protein interactions with the pro-apoptotic protein BIM. These BIM-containing heterodimeric complexes have long been established as hallmarks of primed cancer cells. : Using clinically amenable assay formats, PRIMABs were shown to detect the presence of these anti-apoptotic-pro-apoptotic complexes and their disruption by BH3-mimetic drugs. Moreover, PRIMABs were able to detect a shift in priming status following BH3-mimetic treatment, a factor associated with resistance to these drugs. In a panel of AML patient samples, we report a wide range of priming levels for each PRIMAB complex, demonstrating the potential for heterogeneity in responses. We also show that PRIMABs could be predictive of outcomes for AML patients following cytarabine-based treatment. : PRIMABs provide novel and useful tools for cancer research and for clinical implementation as reagents providing predictive tests for treatment response.