Deleted ring chromosome 22 in a mentally retarded boy.

A mentally retarded boy with a ring chromosome 22, where most of band q13 was deleted, is reported. The fact that the leucocyte beta-galactosidase and alpha-galactosidase B activities were normal, but the arylsulphatase A activity only half of the normal is consistent with a gene dosage effect and that the arylsulphatase A locus is located more distally, than the gene loci for the other two enzymes, in the deleted part of 22q13.