Endoglin is an integral membrane glycoprotein that binds TGF-beta(1,3) with high affinity and is thought to play an important role in modulating the interaction of TGF-beta with its cell surface receptors. In this study a recently characterized monoclonal antibody (29-G8) recognizing endoglin was used to examine expression in a variety of human tissues and human cancer cell lines. Formalin-fixed, paraffin-embedded sections were examined by light microscopy and cell lines were analyzed by now cytometry. Immunostaining was noted in a variety of non-neoplastic epithelia from different organs; most of the neoplastic tissues surveyed also demonstrated prominent immunoreactivity for 29-G8. Flow cytometric analysis of the cell lines revealed strong 29-G8 immunoreactivity in almost all lines examined. Our results suggest that endoglin expression is much more ubiquitous than was previously thought and that endoglin may play a role in modulating TGF-beta binding activity in a variety of normal and neoplastic human tissues.

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