Principal components analysis of array data. The mean expression values of all samples from all arrays were analyzed by principal components analysis. The first 3 principal components of the analysis are shown from the best vantage point to show separation of the three classes. Open circles represent the parental cell lines, "X" denotes the various xenograft tumors, and the small solid dots are the reference cDNA sample (derived from the Universal RNA) co-hybridized with all experimental samples. The cell lines corresponding to the various tissue sources of the parental cell lines were: Ovary, SKOV3; Prostate, PC3; Breast, MDA MB-231; Colon, HCT116; and Lung, A549.