Concordant congenital malformations in twins with inherited translocation: t(9p--;13q+).

Several members of a family with a translocation between the short arm of chromosome 9 and the long arm of chromosome 13(9p--;13q+) are presented. Although the translocation found in various members of the family looked alike and appeared to be balanced, the clinical features were different. The like-sex twins displayed some features of 9p monosomy syndrome, whereas their mother and maternal grandmother, who apparently had the same translocation, showed only a few features of 9p-- syndrome in addition to mild mental retardation. We suggest that a minute deletion of the short arm of chromosome 9 may cause features of 9p-- syndrome and that the clinical features of this syndrome in older individuals may be too mild for the clinical diagnosis to be possible.