• X inactivation
• X
• autosome translocation
• trisomy 10

• LINE-1, long interspersed nuclear element-1
• PCR, polymerase chain reaction
• RT-PCR, reverse transcriptase PCR

Trisomy 10 has been detected in spontaneous abortions and prenatally.¹ Although there are no reports of duplication of the whole long arm of chromosome 10, duplication of 10q21–qter has been found in a stillborn infant.² Trisomy of more distal 10q is associated with a characteristic syndrome and has been described in many cases which almost always are familial,^3,4 but patients with trisomy of the proximal or medial segment of 10q have been less often described.⁴

We report on a 4 year old girl with a de novo unbalanced X;10 translocation, karyotype 46,X,der(X)t(X;10) (q21.2;q11.2), who is thus trisomic for almost the whole long arm of chromosome 10. As her clinical phenotype is disproportionally mild in relation to the region of 10q trisomy, we hypothesised that preferential inactivation of the derivative X;10 chromosome with spreading of X inactivation into the 10q segment might account for this discrepancy between phenotype and genotype. We report results of investigations of late replication, a feature of the inactive X, and expression and sequence analysis of genes located on the translocated region of 10q, and discuss our results relative to previous reports of unbalanced X;10 translocations.