Expression cDNA library of fibroblast growth factor (FGF)-responsive mouse breast cancer cells (SC-3) was prepared and screened using chick FGF receptor (FGF-R) cDNA as a probe. Two positive clones were isolated. Sequence analysis revealed that two clones have an identical full open reading frame. Compared with sequence data on mouse brain FGF-R, SC-3 cells were found to contain FGF-R with 12 amino acids insertion near N-terminal region. This insertion was mainly composed of hydrophobic amino acids. Additionally, the deletion of two amino acids in extracellular domain and the substitution of one amino acid in C-terminal region were identified in SC-3 cell FGF-R. Transfection of this clone into CHO cells resulted in a significant increase in basic FGF (bFGF) binding.