Although a clear genotype–phenotype correlation has not been established, truncating variants (nonsense or frameshift variants, as well as large deletions or insertions) correlating with an absence of functional protein are largely associated with the severe phenotype of classic MD, while variants that permits some residual ATP7A protein activity (including some splice-site, intronic, and missense variants) result in atypical MD or OHS [7,13]. The gene discussed is ATP7A; the disease is Menkes disease.