Address for offprints and correspondence: P.H. Watson, Department of Pathology, Faculty of Medicine, University of Manitoba, 770 Bannatyne Ave., Winnipeg, Manitoba, R3E OW3, Canada; Tel: 204-789-3435; Fax: 204-774-9018; E-mail: pwatson@umanitoba.ca Breast Cancer Research and Treatment 43: 165–173, 1997.  1997 Kluwer A cademic Publishers. Printed in the Netherlands. Report CD 44 variant expression and estrogen receptor status in breast cancer A. Kate Hole, Abbes Belkhiri, Linda S. Snell, and Peter H. Watson Departments of Pathology and Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada, R3E OW3 Key words: breast cancer, CD44, estrogen receptor Summary To understand the relationship between CD44 gene expression and an established variable associated with aggressive behaviour in human breast cancer, we have studied a panel of 6 breast cell lines and 40 breast tumors selected primarily on the basis of estrogen receptor (ER) status. CD44s (standard form) mRNA was assessed by semi-quantitative RT-PCR, and CD44 variants incorporating exon v7 or v10 were studied by RT-PCR and Southern blot. While CD44 expression was not influenced by estrogen in ER+ve MCF-7 cells, CD44s expression was slightly higher (up to 2 fold) in ER -ve cells but there was a marked decrease in the range of CD44 variants incorporating exons v7 or v10. In microdissected tumors, the levels of CD44s showed no correlation with ER status but the pattern of expression of larger forms of CD44 incorporating variant exons v7 and v10 was significantly different (p = 0.005 and p = 0.015, respectively) between ER+ve and ER -ve tumors, reflecting the pattern seen in the cell lines. These findings suggest that the profile of CD44 expression in breast cancer may reflect cellular differentiation as indicated by the ER phenotype. The influence of these differences in CD44 expression on the increased metastatic potential of ER negative breast cancer remains to be determined. Introduction Improvement in the clinical management of breast cancer is in part dependent on the exploration of the importance of known prognostic factors, but al- so on the identification of new ones which will im- prove our ability to assess biological behaviour of tumors. In particular, new factors are needed that might predict the ability of early breast cancer to invade and metastasise. The CD44 proteins are a group of cell surface gly- coproteins that are expressed in a wide variety of tissues and cell types [1]. CD44 has been implicated in a range of biological processes which include cell adhesion [2, 3], migration [4], and metastasis [5] in animal tumor models and recent studies have shown that altered expression of CD44 can also oc- cur in several types of human tumor including breast carcinomas [6]. Furthermore, these alter- ations correlate with tumor progression and metas- tasis, indicating that CD44 may have considerable potential as a diagnostic and prognostic marker [7]. To begin to understand the relationship between CD44 and other biological parameters associated with aggressive behaviour in human breast cancer, we have examined CD44 mRNA expression by RT- PCR in a panel of cell lines and in frozen sections from breast tumor specimens in relation to estrogen