Cell cycle effects on microarray results. A- Hierarchical clustering of genes in different conditions; B- Principal component analysis (PCA). Both methods were performed in GeneSpring on a list of genes that had good confidence measurement and revealed a cell cycle response in gene expression profiles. Before BaP (2.5 μM) treatment for 12 h, MCF-7 cells were synchronised in different phases of the cell cycle. In hierarchical clustering (A), red colour denotes up-regulation and green denotes down-regulation. In PCA analysis (B), squares with black dots denote BaP-treated samples.