[Attenuated phenotype of trisomy 4p by translocation t(X;4)(p21.2;p13)]

A 9-year-old girl with trisomy 4p and monosomy Xp by malsegregation of a maternal translocation t(X;4)(p21.2;p13) was found to have less severe clinical abnormalities and mental retardation than expected for a 4p trisomy syndrome. Height and weight were within the normal range. X replication was studied by BrdU incorporation: in the mother, the normal X was late replicating except in a few cells in which the translocation X was late. In all cells studied from the proband, the translocation X was late, and this late replication extended into the adjoined 4p segment.