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The expression of fibronectin (FN) isoforms including extra domain A (EDA) and extra domain B (EDB) segments, was investigated in 36 invasiveMany recent studies have demonstrated that FN is upregulated in cancer tissues, with reappearance of alternatively spliced isoforms. [6][7][8][9] In breast cancers, immunohistochemical studies using specific monoclonal antibodies have demonstrated that FN isoforms, including EDA (EDA + FN) and EDB (EDB + FN) can be detected in cancer stroma. [10][11][12] In contrast, it has been considered that the effect of the splicing may be quantitative rather than qualitative, 13) and the functional significance of molecular differences has yet to be clarified in detail. However, recent studies have demonstrated that EDA + FN exhibits a high affinity for integrin, 14) and EDB + isoform induces tyrosine phosphorylation of focal adhesion proteins. 15)To establish the possible roles of different FN isoforms in cancer progression, it is important to clarify the cellular sources of these isoforms. Therefore, we examined mRNA expression of EDA and EDB, and well as total FN isoforms, by in situ hybridization (ISH) using cRNA probes specific to the constant region and alternatively spliced sites, in benign and malignant human breast lesions. MATERIALS AND METHODSTissues A total of 49 human breast tumors (36 invasive ductal carcinomas and 13 benign tumors) were investigated. All tissues were surgically resected, and immediately fixed in 4% paraformaldehyde in 0.1 M phosphate buffer (PB), pH 7.4, at 4°C overnight. After having been rinsed in PB, they were dehydrated through a graded ethanol series and xylene, embedded in paraffin, sectioned at 4 µm, and placed on silane-coated glass slides (Dako Japan, Kyoto).Histological classification was made using sections stained with hematoxylin and eosin. To examine the relationship of expression of FN isoforms to morphological

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