Abstract Body: Fetal CMs are characteristically diploid and regenerative, while adult CMs are polyploid and nonregenerative. Increased polyploidy in postmitotic CMs remains a significant barrier to regeneration in the adult heart. Previously, through the analysis of multiple mouse strains, our lab has shown that Tnni3k, a cardiac-specific kinase, increases polyploidy in the adult heart. The mechanisms that induce CM polyploidization are unknown but could be due to a multitude of changes including the postnatal increase in reactive oxygen species (ROS). ROS is essential in cardiac development and works to promote maturation and facilitate cell signaling. Using HEK293 cells, I have found that Tnni3k physically interacts with Prdx2, a prominent cytoplasmic ROS sensor. This interaction results in hyperactivated Tnni3k kinase activity. Comparing Tnni3k kinase activity with interaction of WT or catalytically dead Prdx2 showed hyperactivity dependent on Prdx2 catalysis. Polyploidization increases along with the increase in ROS after birth, and Tnni3k contributes to increased ploidy in mice. I hypothesize that Tnni3k mediates postnatal ROS through Prdx2 signaling to induce polyploidization in postnatal CMs. Discovering the consequences of this interaction could lead to advanced knowledge of CM cell cycle and provide new therapeutic targets.