ABSTRACT: INTRODUCTION: The re-emergence of the tumour growth factor-beta (TGF-beta)-related embryonic morphogen Nodal has recently been reported in several different human cancers. In this study, we examined the expression of Nodal in a series of benign and malignant human breast tissues to determine the clinical significance of this expression and whether Nodal could represent a potential therapeutic target in breast cancer. METHODS: Tissue sections from 431 therapeutically naive patients diagnosed with benign or malignant breast disease were stained for Nodal by immunohistochemistry and analysed in a blinded manner. The degree of Nodal staining was subsequently correlated with available clinical data, such as diagnoses and disease stage. These tissue findings were further explored in breast cancer cell lines MDA-MB-231 and MDA-MB-468 treated with a Nodal blocking antibody to determine biological effects for target validation. RESULTS: A variable degree of Nodal staining was detected in all samples. The intensity of Nodal staining was significantly greater in undifferentiated, advanced stage, invasive breast cancer compared with benign breast disease or early stage breast cancer. Treatment of human breast cancer cells in vitro with Nodal blocking antibody significantly reduced proliferation and colony-forming ability in soft agar, concomitant with increased apoptosis. CONCLUSIONS: These data suggest a potential role for Nodal as a biomarker for disease progression and a promising target for anti-Nodal therapy in breast cancer.