The sixth member of the fibroblast growth factor gene family was cloned and analysed in the mouse. It is composed of three coding exons and encodes a putative growth protein of 198 amino acids, possessing a potential signal peptide, and presenting 79% and 93.5% sequence similarity with the mouse Hst/K-fgf and human FGF-6 genes products, respectively. The murine Fgf-6 gene is located in a region distinct from the Int-41 locus and belongs to a linkage group conserved between chromosome 12 in man and chromosome 6 in mouse. It presents an intrinsic oncogenic capacity since it is able to transform cultured fibroblasts. Fgf-6 mRNA levels are developmentally regulated with a peak of expression in the developing fetus at day 15.5 of gestation, moderate levels during late gestation and in the neonate. In the adult, Fgf-6 mRNA can be detected in testis, heart and skeletal muscle.