Principal component analysis of piriform cortex samples. PCA of gene expression profiles reveals partitioning of piriform cortex samples based on sarin-induced seizure occurrence and time point following seizure onset. Tissues were isolated at 0.25, 1, 3, 6, and 24 h after seizure onset and processed for oligonucleotide microarray analysis. The raw signal intensities were normalized using the RMA algorithm and visualized using PCA to identify major sources of variability in the data. Each point on the PCA represents the gene expression profile of an individual animal. Point shape corresponds to exposure condition, point color corresponds to the time after seizure onset at which the tissue was collected, and point size indicates absence or occurrence of sarin-induced seizure. The principal components in the three-dimensional plot represent the variability in gene expression levels seen within the dataset: PC#1 (x-axis) accounts for 21.70% of the variability in the data; PC#2 (y-axis) represents 16.20% of the variability; and PC#3 (z-axis) represents 6.26% of the variability in gene expression levels seen within the dataset.