FZD9 (frizzled class receptor 9) is a Wnt receptor that plays diverse roles in development, tissue homeostasis, and disease. FZD9 exhibits widespread expression during embryonic development in neural tube, skeletal muscle precursors, limb skeletal structures, craniofacial regions, and nephric ducts, with abundant expression in adult heart, brain, testis, and skeletal muscle 1. The protein functions as a membrane-bound receptor involved in both canonical and non-canonical Wnt signaling pathways 1. In cancer contexts, FZD9 is upregulated in astrocytomas, with expression levels correlating positively with tumor grade and proliferative activity 2. FZD9 also shows elevated expression in osteosarcoma tissues and cell lines, where it works alongside Wnt2 and c-Fos in promoting cell proliferation, migration, and invasion 3. Additionally, FZD9 expression is regulated by miR-520a-5p, with cigarette smoke increasing this microRNA and consequently reducing FZD9 levels, while the chemopreventive agent iloprost has opposite effects 4. FZD9 is considered a stemness marker, with its expression being upregulated during angiogenic induction of stem cells but decreasing with serial passage culture 56. The gene is located in the Williams-Beuren syndrome deletion region, potentially contributing to the cognitive and behavioral phenotype of this neurodevelopmental disorder 7.