RSPO4 is a secreted ligand that activates canonical Wnt signaling by binding to LGR4-6 receptors and antagonizing the E3 ubiquitin ligases ZNRF3 and RNF43 1. Upon LGR4-6 binding, RSPO4 promotes ternary complex formation with phosphorylated LRP6 and frizzled receptors, enabling Ξ²-catenin nuclear translocation and TCF/LEF-dependent gene transactivation 2. However, RSPO4 has the lowest binding affinity for ZNRF3/RNF43 and weakest efficacy among R-spondins, making it functionally distinct from RSPO1-3 3. In nail development, RSPO4 is expressed in onychofibroblasts proximal to LGR6+ nail matrix cells, activating Wnt/Ξ²-catenin signaling essential for nail specification 4. Biallelic RSPO4 mutations cause congenital anonychia/hyponychia, with mutations clustering in the highly conserved furin-like cysteine-rich domains 56. Beyond nail development, RSPO4 variants associate with severe periodontitis through effects on innate immune response and barrier integrity 7. Notably, RSPO4 exhibits tumor suppressor activity through antagonizing both canonical and non-canonical Wnt signaling, with frequent epigenetic silencing in multiple cancer types 8.