Oestrogen-induced pS2 protein is similar to pancreatic spasmolytic polypeptide and the kringle domain.

are oestrogen-dependent (Katzenellenbogen, 1980; McGuire, 1980). The MCF-7 cell line (Lippman & Bolan, 1975), a human breast cancer cell line, has been especially useful for understanding the mechanism of action of oestrogen. The pS2 protein (Mr 6500) is an abundant oestrogen-induced protein secreted by MCF-7 cells and found in breast tumour tissue (Masiokowski et al., 1982; Jakowlew et al., 1984; Prud'Homme et al., 1985; Nunez et al., 1987), but not in non-malignant breast tissue. pS2's function is unknown. Using computer-based analyses of amino acid sequences, we find that: (1) pS2 is homologous to the pig pancreatic spasmolytic polypeptide, which represses gastrointestinal motility (spasmolytic effect) and inhibits pentagastrininduced gastric acid secretion (J0rgensen et al., 1982; Thim et al., 1985; Frandsen et al., 1986), and (2) both pS2 and pancreatic spasmolytic polypeptide are similar to the kringle domain, found in the non-catalytic region of plasminogen and several other serine proteases important for blood coagulation and fibrinolysis (Doolittle, 1985; Patthy, 1985; Young et al., 1978). These similarities should provide clues for further studies to understand the role of pS2 in the growth of oestrogendependent tumours, as well as the functions of pancreatic spasmolytic polypeptide and the kringle domain. The similarity between pS2 and pancreatic spasmolytic polypeptide was arrived at as a result of our interest in