Tandem duplication dup(X)(q13q22) in a male proband inherited from the mother showing mosaicism of X-inactivation.

An aberrant X chromosome containing extra material in the long arm was observed in a psychomotoric retarded boy and his healthy, short-statured mother. The proband showed generalized muscular hypotony, growth retardation, and somatic anomalies including hypoplastic genitalia and cryptorchism. Chromosomal banding techniques suggested a tandem duplication of the segment Xq13 leads to Xq22. In the mother the vast majority of lymphocytes showed late replication of the aberrant X chromosome. Some of her cells, however, contained an apparently active aberrant X. Both the early- and late-replicating aberrant X exhibited late replication patterns very similar to those described for normal X chromosomes in lymphocytes. Asynchrony of DNA replication among the two segments Xq13 leads to Xq22 in the dup(X) was never observed. We consider that the clinical picture of the proband is caused by an excess of active X material.