Heatmap of k-means cluster analysis of 214 differentially expressed miRNAs in 88 rat liver samples. Normalized log2 intensity values for 214 miRNAs that were differentially expressed by age and/or sex in liver of rats of nine ages and both sexes were used for k-means cluster analysis where k = 10 clusters. Intensity scale is shown on the upper right with red indicating high expression and black indicating low expression. Clusters are identified on the left and miRNAs identified in Additional file 2. Age (in weeks) is along bottom.