PRND encodes the doppel protein (Dpl), a prion-like protein with dual roles in reproduction and potentially in vascular pathology. Primary Function: PRND is required for normal acrosome reaction and male fertility 1, with demonstrated involvement in testis development across mammalian species. The protein localizes to the external plasma membrane and can bind copper ions [UniProt sources cited in abstracts]. Mechanism: Doppel functions in cellular copper ion responses and interacts with binding partners through protein-protein interactions, though detailed molecular mechanisms remain incompletely characterized. Disease Relevance: While PRND polymorphisms show variable associations with prion diseases across populations—with significant genotype frequency differences at codon 174 in some but not all populations 234—a recent discovery demonstrates PRND's upregulation in pulmonary arterial hypertension (PAH) endothelial cells, where doppel blocking activates the BMPRII/pSMAD1/5 pathway and reduces endothelial-to-mesenchymal transition markers 5. Clinical Significance: PRND polymorphisms identified in multiple species show strong genetic linkage with prion protein gene (PRNP) variants 67, suggesting potential utility as ancestry markers. The emerging role in PAH pathogenesis represents a novel therapeutic target, with doppel-knockout mice showing reduced right ventricular systolic pressure in disease models 5.