A marker chromosome present in all the lymphocytes of a mentally retarded boy (47,XY,+mar) was identified. The karyotype of all fibroblasts was normal, 46,XY. His parents had normal karyotypes. The supernumerary marker chromosome was interpreted as a dicentric palindromic chromosome iso(9) (pter----q12, q12----pter). The marker chromosome may have originated through a ligation error in a replication fork during the S-phase followed by a new replication (endoreduplication).