Heat map analysis of microarray data showing differentially expressed genes between mature and old single R15 neurons. (A) Principle Component Analysis (PCA) mapped scatter plot: The global gene expression profiles of the R15 neurons from both mature and old Aplysia were analyzed by PCA. The figure represents the first two principal components of microarray analysis data (PC1, PC2 and PC3) in X, Y and Z respectively, and demonstrated the expression profile of the three single neurons per group (Mature Red # 1–3, Old blue# 1–3). (B) Heat map analysis of microarray data showing hierarchical clustering of 1083 differentially expressed probes between R15 mature and old single neurons. Each group has three single neurons tested. Red or green colors indicate differentially up or downregulated genes, respectively. The mean signals were background corrected and transformed to the log2 scale. Genes with at least 2-fold changes with p < 0.05 at the 95% confidence level were considered as significant.