THY1 (Thy-1 cell surface antigen) is a cell surface glycoprotein that functions as a marker of distinct fibroblast and progenitor cell populations with specialized roles in tissue homeostasis and disease pathology. In synovial tissue, THY1 expression distinguishes fibroblast subsets with non-overlapping functions: FAPα+THY1+ fibroblasts in the synovial sub-lining mediate inflammatory responses, while FAPα+THY1- fibroblasts in the lining layer drive bone and cartilage destruction in arthritis 1. Single-cell analyses reveal THY1 marks sublining connective tissue fibroblasts of mixed embryonic origin and serves as a lineage marker during fibroblast differentiation from homeostatic Thy1+ states toward destructive Thy1- identities in arthritic disease 23. In neural development, THY1 expression stratifies neural stem and progenitor cells: CD24-THY1-/lo cells are enriched for radial glia, while THY1hi cells mark oligodendrocyte precursors, enabling prospective isolation of distinct developmental lineages 4. In diabetic kidney disease, THY1 expression in fibroblasts is associated with antifibrotic protective properties, with high diagnostic accuracy for fibrosis assessment 5. These findings establish THY1 as a critical surface marker distinguishing functionally discrete cell populations across multiple tissue contexts.