The POU domain transcription factor Oct4 (POU5F1 or Oct3) is a member of the homeobox gene family and is specifically expressed in totipotent mouse and human embryonic stem (ES) and germ cells.It is considered to be a gatekeeper in the early steps of mammalian embryogenesis because of its pivotal role in the regulation and maintenance of pluripotency and self-renewal of ES cells.Expression of Oct4 is down-regulated when embryonic stem cells are triggered to differentiate and is lost in normal somatic cells of differentiated tissues.However, in our previous study expression of Oct4 mRNA was detected in all 28 primary breast tumors examined irrespective of their types (adenocarcinoma, fibroadenoma or ductal in-situ carcinoma) and their expression levels were thousand fold higher compared to normal tissues.In addition, the Oct4 mRNA levels in individual tumor tissues had a strong positive correlation with ER status (P < 0.01).In this study, we further examined Oct4 protein levels in breast tumors and normal breast tissues by Western blotting analysis and immunofluorescence staining.Oct4 protein could be detected in all tumors examined but was barely detectable in normal breasts.Furthermore, sequencing analysis of Oct4 transcripts in breast tumors, breast cancer cell line MCF7 cells, normal breast tissues and normal breast cell line MCF10 cells showed that the normal breast tissue and MCF10 cells express multiple transcripts of the Oct4 gene on chromosome 6, but these transcripts could not be translated.On the other hand, the breast tumors and MCF7 cells express multiple transcripts of an Oct4-like gene on chromosome 8 that we term Oct3.The deduced amino acid sequence of the full length Oct3 cDNA is 96% identical to the Oct4 protein.Our study indicates that breast cancer cells may aberrantly express an Oct4-like gene (Oct3), but not Oct4 gene and Oct3 may be a novel marker of breast cancer.