The study aimed to identify and characterize a fibroblast-specific protein, fibroblast-specific protein 1 (FSP1). Through subtractive and differential hybridization, the researchers identified FSP1 as a gene highly specific to fibroblasts. FSP1 is expressed in fibroblasts but not in epithelial cells, mesangial cells, or embryonic endoderm. During development, FSP1 is first detected after day 8.5 and is associated with mesenchymal cells or fibroblasts. Polyclonal antibodies against recombinant FSP1 stained fibroblasts but not epithelial cells in normal tissues. However, in kidneys with fibrosis, many fibroblasts were identified in areas of collagen deposition and adjacent to inflammatory processes. This suggests that FSP1 may be involved in the conversion of epithelial cells to fibroblasts under certain conditions. In vitro experiments showed that overexpression of FSP1 in tubular epithelial cells led to a fibroblast-like phenotype, including increased expression of vimentin and reduced cytokeratin. Additionally, tubular epithelial cells submerged in type I collagen gels converted to a fibroblast phenotype, de novo expressing FSP1 and vimentin. The FSP1 marker is expected to facilitate studies on fibrogenesis and cell fate mapping in fibroblasts.The study aimed to identify and characterize a fibroblast-specific protein, fibroblast-specific protein 1 (FSP1). Through subtractive and differential hybridization, the researchers identified FSP1 as a gene highly specific to fibroblasts. FSP1 is expressed in fibroblasts but not in epithelial cells, mesangial cells, or embryonic endoderm. During development, FSP1 is first detected after day 8.5 and is associated with mesenchymal cells or fibroblasts. Polyclonal antibodies against recombinant FSP1 stained fibroblasts but not epithelial cells in normal tissues. However, in kidneys with fibrosis, many fibroblasts were identified in areas of collagen deposition and adjacent to inflammatory processes. This suggests that FSP1 may be involved in the conversion of epithelial cells to fibroblasts under certain conditions. In vitro experiments showed that overexpression of FSP1 in tubular epithelial cells led to a fibroblast-like phenotype, including increased expression of vimentin and reduced cytokeratin. Additionally, tubular epithelial cells submerged in type I collagen gels converted to a fibroblast phenotype, de novo expressing FSP1 and vimentin. The FSP1 marker is expected to facilitate studies on fibrogenesis and cell fate mapping in fibroblasts.