Mammary-derived growth inhibitor (MDGI), an inducer of rodent mammary differentiation and suppressor of human breast cancer cell growth, has been cloned from bovine and rodent mammary glands. The present study reports the cloning of MDGI from cultured human breast epithelial cells (HBEC-MDGI) as a cDNA fragment encoding a protein of 133 amino acids identical to heart fatty acid binding protein. Expression of HBEC-MDGI, as detected by in situ hybridization in paraffin-embedded normal breast tissues, was maximal in the most differentiated lobules type 4, low in the moderately differentiated lobules type 3, and absent in the least differentiated lobules types 1 and 2. HBEC-MDGI was not expressed in breast tissues that contained ductal hyperplasia, carcinoma in situ or invasive carcinomas. Our results indicate that HBEC-MDGI is a biomarker of lobular differentiation in the human breast, and its expression is silenced in poorly differentiated lobules as well as in the early and late stages of breast cancer progression.

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