Senger et 01.'in 1979 reported that virally and spontaneously transformed cell lines derived from several mammalian species secrete a major phosphoprotein with an approximate molecular mass of 60 kDa (transformation-specific phosphoprotein). Later, in 1985, Franzen and Heinegard2 isolated a major 57-kDa sialoprotein from calcified matrix of rat bone, 3 and Chackalaparampil et~ 1.~ reported that normal rat kidney (NRK) cells in culture secrete a 69-kDa major phosphoprotein (pp69). Oldberg et al.* in 1986 isolated the cDNA clone of the sialoprotein identified by Franzen and Heinegard2 from a rat osteosarcoma (R07 17/2.8) phage Agtll library. Analysis of the amino acid sequence deduced from the cDNA revealed a GRGDS cell-binding sequence and likely sites for N-and 0-glycosidically linked oligosaccharides. Purified protein also showed cell attachment activity. They named this protein" osteopontin." In 1987 Prince et alSs independently purifed and characterized a 44-kDa phosphorylated glycoprotein from rat bone which is also synthesized and secreted by ROS17/2.8 cells. Denhardt et al. in 1988 showed that mouse 2ar, a tumor promoter-inducible protein secreted by mouse JB6 epidermal cells is homologous to rat OPN. Craig et a1.* in the same year noted that the sequence of nine N-terminal amino acids of the rat transformation-specific phosphoprotein reported by Senger et aL9 is identical to the N-terminal 17 to 25 amino acid residues of the polypeptide deduced from the cDNA sequence of rat a This work was supported by grants from the Medical Research Council, Canada (MT-11628 to BBM). SB was the recipient of a professional services contract from the National Institute of Child Health and Human Development, NIH. Present address: Cardiovascular Division, Brigham Woman's Hospital, Boston, MA. To whom correspondence should be addressed.