Hormones and their receptors play an important role in the development and progression of breast carcinoma. Although the primary focus has been on oestrogen and oestrogen receptor (ER), androgen, androgen receptor (AR) and its coactivator(s) have been implicated in tumorigenesis of breast carcinoma and warrant further investigation. AR coactivator p44/Mep50 is identified as a subunit of methylosome complex and lately characterized as an AR coactivator that enhances AR mediated transcription activity in a ligand dependent manner. In prostate cancer, p44 is expressed in the nucleus of benign epithelia and translocated into the cytoplasm in cancer cells. Furthermore, nuclear expression of p44 inhibits prostate cancer growth. In this report, we examined the expression and function of p44 in breast cancer. In addition to being an AR coactivator, p44 also functions as an ER coactivator. In contrast to findings in prostate cancer, the expression of p44 shows strong cytoplasmic expression in morphologically normal terminal ductal lobular units, while nuclear p44 is observed in both ductal carcinoma in situ and invasive carcinoma. Further, overexpression of nuclear-localized p44 stimulates proliferation and invasion in MCF7 breast cancer cells in the presence of oestrogen and the process is ERα dependent. These findings strongly suggest that p44 plays a role in mediating the effects of hormones during tumorigenesis in breast.