A novel 85-kDa protein secreted by the mouse stromal osteogenic cell line MN7 was identified using two-dimensional polyacrylamide gel electrophoresis (Mathieu, E., Meheus, L., Raymackers, J., and Merregaert, J. (1994) J. Bone Miner. Res. 9, 903-913). Degenerate primers were used to isolate the cDNA coding for this protein. The full-length cDNA clone is 1.9 kilobases (kb) and codes for a protein of 559 amino acid residues. The DNA and deduced amino acid sequences have no counterparts in public data bases, but a structural similarity involving typical cysteine doublets can be observed to serum albumin family proteins and to Endo16 (a calcium-binding protein of sea urchin). Northern blot analysis revealed the presence of a 1.9-kb transcript in various tissues, and a shorter transcript of 1.5 kb, derived by alternative splicing in tail, front paw and skin of embryonic mice. The gene for the p85 protein, termed Ecm1 (for extracellular matrix protein 1), is a single-copy gene, which was localized to the region on mouse chromosome 3 known to contain at least one locus associated with developmental disorders of the skin, soft coat (soc). Alternative splicing may serve as a mechanism for generating functional diversity in the Ecm1 gene.