Basic regulation types in combinatorial gene regulatory network. TFs and miRNAs are two different types of gene expression regulators that can regulate protein-coding genes (termed "Target (gene)" herein), and they can regulate each other as well. TFs regulate protein-coding genes or miRNA genes at the transcription level (TF→Target (gene) and TF→miRNA, solid lines), while miRNAs regulate protein-coding genes at the post-transcription level (miRNA→Target (gene) and miRNA→TF, dotted lines). In this figure, the same node"Target (gene)" is used to signify both the transcription level and the post-transcription level. As TFs are in fact protein-coding genes, miRNA→target regulations and miRNA→TF regulations can be grouped together into one type miRNA→Target(gene). Overall, the regulations in combinatorial gene regulatory networks are classified into three types: TF→miRNA, miRNA→Target (gene), and TF→Target (gene).