46,XY del(18)(q21.3q22.2) with mosaicism of r(18) and a milder form of the 18q- syndrome.

A mosaic karyotype: 46,XY,del(18)(q21.3q22.2)/47,XY,del(18) (q21.3q22.2)+ marker, was found in a mentally retarded male with a mild form of the 18q- syndrome and aplasia of the right thumb. By fluorescent in situ hybridisation, the marker chromosome could be identified as a ring chromosome no. 18.