As shown in Figure 3B, the co-cited references were grouped into twenty-seven major clusters (lateral habenula, dopamine, DeltaFos-B, epithalamus, habenula, Parkinson’s disease, depression, vesicular acetylcholine transporter, nucleus accumbens, c-Fos, nicotine, evolution, schizophrenia, diencephalon, CHRNA5, DBS, zebrafish, θ oscillations, GCaMP, receptor autoradiography, epibatidine, a-adrenoceptors, corticohabenular, lateralization, glutamate neurons, immunohistochemistry, and tail of the ventral tegmental area). The gene discussed is FOS; the disease is depressive disorder.