A nucleic acid sequence encoding Mammastatin, a specific mammary cell growth inhibitor. Mammastatin is encoded by a single nucleic acid sequence and has an approximate molecular weight of 44 kDa in its inactive, non-phosphorylated form. Normal mammary cells express functional phosphorylated forms having approximate molecular weights of 53 kDa and 49 kDa. Metastatic mammary cells either do not express Mammastatin at all, or do not express the 53 kDa or 49 kDa, phosphorylated forms. Mammary cancer cells are inhibited in their growth by the administration of phosphorylated mammastatin.