Immunohistochemical comparison of different YB-1 antibodies in human invasive breast cancer. While the proliferation marker Ki67 is strongly expressed in a defined fraction of breast tumor cells there is no concordance to the expression pattern of YB-1. The three YB-1 antibodies present very similar expression patterns. However, while the two established polyclonal YB-1 antibodies (#1 is against an N-terminal epitope and available from antibodies-online; #2 has been established by Janz et al. ) predominantly detect cytoplasmic YB-1 expression, the newly characterized monoclonal YB-1 antibody Mab F-E2G5 is able to detect nuclear YB-1 besides less abundant expression in the cytoplasm.