RASSF9 (Ras association domain family member 9) is a regulatory protein with emerging roles in epithelial homeostasis and cell polarity. RASSF9 functions as a modulator of PAR3 condensates, promoting phase separation to maintain apical-basal polarity in epithelial tissues 1. The protein plays a critical role in epidermal homeostasis; RASSF9-null mice exhibit disrupted epithelial organization with increased keratinocyte proliferation and aberrant differentiation, accompanied by reduced p21Cip1 expression 2. RASSF9 acts as a tumor suppressor in multiple cancer types. In thyroid cancer, decreased RASSF9 associates with advanced stages and lower disease-free survival, with overexpression inhibiting Wnt/β-catenin pathway activation and suppressing cancer cell proliferation, invasion, and migration 3. In breast and gastric cancers, RASSF9 is targeted by oncogenic microRNAs (miR-1254 and miR-1269), which promote cancer cell proliferation and survival by suppressing RASSF9 expression and activating AKT signaling [PMID:30132526; 40]. Additionally, RASSF9 shows sun-exposure-dependent gene expression regulation in human skin, with allele frequencies correlating with solar radiation exposure across populations, suggesting adaptive significance 5. These findings indicate RASSF9 has diagnostic and prognostic potential in cancer management.