The third window, located on chr5:141,000,000–142,000,000 (q31.3), contains the HDAC3 gene, a key regulator of gingival fibroblast inflammatory responses in periodontitis [15] and RELL2, a homologue and co-regulator of the gene RELT, which causes hypomineralized amelogenesis imperfecta [16], and the protocadherins (PCDHs) gene cluster, a group of cell–cell adhesion molecules that play a role in odontogenesis [17] and are epigenetically regulated [18]. This evidence concerns the gene RELL2 and periodontitis.