The PLU-1 gene is expressed at the level of message in breast cancers and breast cancer cell lines and shows restricted expression in normal adult tissues with the exception of testis. The predicted protein sequence contains several domains, including the PLU domain, which is shared by other proteins involved in transcription and/or development. We have developed a polyclonal antiserum to a C-terminal fragment of the PLU-1 protein, which shows little homology to other family members. Immunohistochemic analysis with the antiserum α-PLU-1C confirmed the nuclear localisation of PLU-1. α-PLU-1C also reacted with the mouse homologue of PLU-1 (mPlu-1) but not with the closest family member, RBP2. Using Western blot analysis, PLU-1 was shown to be well expressed in breast cancers and breast cancer cell lines, while it was not detected in a range of normal adult tissues. Our results suggest that the PLU-1 protein may belong to the class of testis/cancer antigens. © 2002 Wiley-Liss, Inc.