PARD3B (par-3 family cell polarity regulator beta) is a conserved polarity determinant that functions as a junctional adapter protein distinct from its apical-localized ortholog PARD3 1. In epithelial tissues, PARD3B localizes strictly to adherens junctions where it may cluster E-cadherin, coordinate signaling through Src family kinases, and orient mitotic spindles in response to mechanical forces 1. This junctional specialization emerged during vertebrate evolution coinciding with stratified squamous epithelia development, where PARD3B is strongly expressed in basal layer stem cells lacking typical apical domains 1. Beyond its structural roles in cell polarity, PARD3B has pathological significance in multiple diseases. In intrahepatic cholangiocarcinoma, PARD3B mRNA stabilization via the VIRMA-HuR axis activates Akt/GSK/Ξ²-catenin and MEK/ERK signaling, promoting proliferation and metastasis 2. In glioblastoma, testosterone-AR signaling upregulates PARD3B expression to stimulate cell proliferation and colony formation 3. Additionally, PARD3B genetic variants associate with reticular pseudodrusen in age-related macular degeneration 4, hydrochlorothiazide-induced hyperuricemia 5, dementia with Lewy bodies 6, type 2 diabetes susceptibility 7, and HIV immunotherapy response 8, suggesting pleiotropic effects on cellular and systemic physiology.