RCAN3 (Regulator of Calcineurin 3) is a multifunctional protein that primarily inhibits calcineurin-dependent signaling pathways through direct binding to the catalytic domain of calcineurin A 1. The protein contains a conserved PxIxIT motif that competes with NFATc transcription factors for calcineurin binding, thereby inhibiting NFATc dephosphorylation and activation 2. RCAN3 exhibits complex alternative splicing patterns, with at least 21 different isoforms identified that show ubiquitous tissue expression 3. Functionally, RCAN3 regulates cell proliferation in endothelial cells, inhibiting both basal and VEGF-stimulated HUVEC growth independently of inflammatory gene expression 1. The protein also interacts with cardiac troponin I, suggesting roles in cardiac function 4. Disease relevance includes cardiac development, where RCAN3 deficiency contributes to noncompaction of ventricular myocardium, and homozygous missense mutations cause marked cardiac dysfunction 5. In cancer, RCAN3 functions as a tumor suppressor, particularly in triple-negative breast cancer, where it inhibits tumor growth and angiogenesis through calcineurin-NFATc pathway suppression 62. Additionally, RCAN3 negatively regulates calcium-dependent cytotoxicity in CD8+ T lymphocytes 7 and shows potential as a pan-cancer diagnostic and prognostic biomarker 8.