PIBF was previously identified as a 34 kDa immunomodulatory molecule secreted by pregnancy lymphocytes and is thought to play a crucial role in preventing rejection of the embryo by the maternal immune response. Recent data suggested that PIBF protein was also expressed by the progesterone receptor (PR) positive MCF-7 breast tumor cell line. Therefore our study was designed to analyze the expression of PIBF in malignant cell lines and primary tumors both at the mRNA and protein levels. RNA expression analyses of several human cell lines with different tissue origin and paired human tumor/normal tissues, as well as of several PR+ and PR- breast tumors revealed that PIBF mRNA was overexpressed in highly proliferating cells independent of the presence of PR. In addition to the full-length PIBF mRNA encoding for a 90 kDa protein, several alternatively spliced species were detected, all resulting from perfect exon skipping. The most frequently identified splice variant is predicted to encode for an approximately 35 kDa protein. Immunofluorescence microscopy revealed a centrosomal localization for the full-length PIBF, while the 35 kDa form showed a diffuse cytoplasmic staining. These data, together with the identification of the PIBF gene in the chromosomal region associated with breast cancer susceptibility, reveal a strong parallel with known tumor suppressor proteins, such as BRCA1 and p53 having the same centrosomal localization. Given the notion that a number of proteins shown to be involved in tumorigenesis are associated with the centrosome and disturbed centrosome function causes unequal segregation of chromosomes, studies to evaluate whether or not PIBF that is highly expressed in tumors is directly involved in tumorigenesis are thus warranted.