LRRN4 (leucine-rich repeat neuronal 4) is a leucine-rich repeat domain-containing protein with diverse roles across multiple physiological systems. In the nervous system, LRRN4 may play an important role in hippocampus-dependent long-lasting memory according to UniProt annotations. LRRN4 serves as a specific marker of primary mesothelial cells 1, with distinct expression patterns in idiopathic pulmonary fibrosis (IPF), where it is found in epithelial lining cells along honeycombing and fibroblastic foci and correlates with decreased lung function 2. In cardiac tissue, LRRN4 is highly expressed in donor hearts but significantly reduced in dilated cardiomyopathy (DCM), suggesting it is critical for maintaining cardiac morphology and function 3. LRRN4 expression is regulated by cardiac stress responses, with cardiomyocyte-specific MIAT deletion reducing pathological LRRN4 induction after myocardial infarction 4. In cancer biology, LRRN4 appears to function as a tumor suppressor; its knockdown in p53- and Rb-defective ovarian epithelial cells promotes aggressive malignant behavior through Akt pathway activation 5. Additionally, low prenatal socioeconomic status associates with altered LRRN4 DNA methylation at birth, with associations persisting into early childhood 6, and a structural deletion variant in LRRN4 was validated as associated with Parkinson disease risk 7. In neuroprotection studies, LRRN4 was identified as upregulated following aluminum ion-induced neurotoxicity in neuroblastoma cells 8.