These keywords were further classified into 11 clusters as follows: cluster 1 (epidemiology), cluster 2 (placebo-controlled trial), cluster 3 (steroid), cluster 4 (late luteal phase), cluster 5(premenstrual syndrome), cluster 6 (emotional perception), cluster 7 (dysmenorrhea), cluster 8 (therapeutic use), cluster 9 (brain-derived neurotrophic factor), cluster 10 (delta) and cluster 11 (Japanese women) (Figure 7, Table 4). This evidence concerns the gene BDNF and premenstrual tension.