To further examine the mechanistic basis of preconditioning, we investigated whether higher pro-survival gene expression prevents coral from bleaching, by inhibiting the effect of pa-Bcl-2 in PC corals. If the acquired higher thermotolerance is caused by the pro-survival shift in PCD pathway, inhibition of pa-Bcl-2 function should result in the loss of the beneficial phenotype. Venetoclax (ABT-199) is a BH3 mimetic small molecule that was reverse engineered to bind specifically to the Bcl-2 protein binding domain with a very low affinity to other BH3-containing domain proteins, like Bcl-XL and MCL-1 (Souers et al., 2013). It antagonizes the interaction between Bcl-2 and its target pro-death proteins and thus prevents its pro-life function in PCD (Lampson & Davids, 2017). We treated PC and NPC heat-stressed corals with venetoclax to assess the impact of pa-Bcl2 inhibition on bleaching rate.