PURPOSE: The runt-related transcription factor, Runx2 may have an oncogenic role in mediating metastatic events in breast cancer, but whether Runx2 has a role in the early phases of breast cancer development is not clear. We examined the expression of Runx2 and its relationship with oestrogen receptor (ER) and progesterone receptor (PR) in breast cancer cell lines and tissues. METHODS: Two human breast cancer cell lines, MCF-7 and MDA-MB-231 were transiently transfected with vectors expressing either Runx2 or ER and the levels of both proteins and mRNA were examined by Western blot analysis and quantitative real-time PCR, respectively. Runx2 expression was also examined in tissue microarray sections of 123 breast cancer patients by immunohistochemistry and results were correlated with clinico-pathological characteristics. RESULTS: Expression of Runx2 and ER was reciprocal in the breast cell culture models and Runx2 suppressed ERbeta but not ERalpha mRNA levels. In contrast, functional expression of Runx2 was evident in the nucleus in 28% of the breast cancer tissues and in both early and late stages of tumour growth. Importantly, Runx2 expression was significantly more frequent in Grade 2 compared to Grade 1 and Grade 3 tumours (48% versus 39% versus 13%) and the expression was significantly associated with ER (p=0.005), PR (p=0.008) expressions in Grade 2 and Grade 3 tumours than Grade 1 tumours. CONCLUSION: We propose that Runx2, ER and PR triple positivity in Grades 2 and 3 defines a biological subtype in breast cancer.