MT1E and Down syndrome: It is important to highlight that a low gene expression of MT1 and MT2 in Down syndrome patients in our study has been observed when comparing patients with periodontal disease and failed implant treatment with patients who do not suffer from periodontal disease and have a positive experience of implant treatment, but we do not know what expression metallothionein genes would have if another study group with different characteristics of periodontal disease and implant evolution were used.