S1PR4 (Sphingosine-1-Phosphate Receptor 4): S1PR4 is a G protein-coupled receptor for the lipid signaling molecule sphingosine-1-phosphate (S1P). It is primarily expressed in hematopoietic cells and plays a key role in regulating immune cell migration, inflammation, angiogenesis, and endothelial barrier integrity. Alterations in S1PR4 signaling can impact immune responses and contribute to inflammatory and immune-related disorders.