Rare duplications can occur on the short or on the long arm of the X chromosome and may be fa­milial or of de novo origins [1-4]. Most X chromo­some duplications are thought to have a prezygotic origin, as suggested by the absence of a normal cell line and lack of mosaicism [1]. A 1:1 segregation in a female heterozygote is expected to result in male hemizygotes and female heterozygotes who inher- it the dup(X) duplication [2]. A familial inverted duplication of an X chromosome was traced to a grand-parental mitotic error [3]. The importance of pedigree analyses and family studies in determining the origin of such duplications needs no emphasis.

Patients who carry a dup(X) chromosome pro­vide opportunities to evaluate genotype/phenotype correlation in relation to X chromosome gene con­tent and inactivation. Such abnormalities are gener­ally well tolerated in female carriers because of pref­erential inactivation of the abnormal X, which can restore, at least in part, a balanced genetic makeup [4]. In contrast, males with dup(X) which is geneti­cally active, have a functional partial X disomy and are of abnormal phenotype [5]. Here we report on two female patients with 46,X,dup(X)(q13q25) and 46,X, dup(X)(p11.3p21) karyotypes, respectively.