We report on the clinical and cytogenetic assessment of five cases of Down syndrome phenotype with either a partial duplication of chromosome 21 or a normal karyotype, and we quote a case of del (21q) syndrome. Down syndromes with a partial duplication of chromosome 21 (as well as cases of del (21q), which are partly the phenotypic countertype of trisomy 21) are of paramount importance in the understanding of genes involved in the phenotype of Down syndrome. The goal is to find the relevant genes implicated in the main traits of Down syndrome (i.e. mental retardation, Alzheimer disease, and serious visceral malformations). Such a goal, in our opinion, cannot be reached just by publishing the genotype and the phenotype of a small cohort of patients: 1. a sufficient number of accurate cases is needed, and 2. data have to be computerized for definite conclusions to be reached. The main aims of this report are to present our study protocol and to invite colleagues to participate in a collaborative study in order to collect a maximum of these (rare) cases.