The dendrogram for the Jaccard coefficient matrix P among the 23 KEGG pathways on the basis of the five genes and the sorted data matrix B in Fig. 5 reveal that the pathway pair of dopaminergic synapse and neuroactive ligand–receptor interaction (sharing DRD3, DRD2, and GRIN2B) is strongly associated with the pathway group of cocaine addiction, cAMP signaling, and Rap1 signaling (sharing DRD2 and GRIN2B). This evidence concerns the gene DRD3 and cocaine dependence.