When the prognostic value of BUB1, PLK1 and CCNA2 expression was evaluated using publicly available breast cancer microarray datasets (with associated clinical information), we found that high expression levels of all three genes are predictive of lower disease-free survival in the datasets from Ivshina et al. (Ivshina et al, 2006) and Miller et al. (Miller et al, 2005; Fig 5A). This evidence concerns the gene PLK1 and breast cancer.