Received December 27, 2010
Accepted January 10, 2011

We documented for the first time a cryptic unbalanced chromosomal rearrangement 46, X, der (X) t (X; 18) (q27.2, q22.1) in a phenotypically normal woman. The karyotype with high resolution banding showed a normal karyotype. The study of the triplet CGG of the FMR1 gene (Fra-X) showed an only one allele, suggesting a deletion in the distal portion of Xqter. The FISH study with Tel Xq probe showed a single signal. The microarray-CGH also showed a deletion from Xq27.2 to Xqter, and a partial trisomy from 18q22.1 to 18qter. The FISH study with whole painting probes of chromosomes X and 18 and telomeric 1􀀛q probe confirmed the non-reciprocal translocation X;18. The QF-PCR with STRs linked to distal part of Xqter (DXS1205, DXS8091, DXS8069, DXS1684 and DXS15) also corroborated the Xq deletion The proband is the first child of a brotherhood of four. We studied the sister, the twins brothers, mother and grandmother. The preferential X inactivation was performed. The three carriers of the der (X) had preferentially inactivated the der (X). It’s well recognized the the study of the premutation Fra-X and its association with ovarian failure. The finding of a single allele of the CGG triplet is indicative of Xqter deletion, region where also map POF1 genes. We discuss the different behavior of the same chromosome abnormality in three women carrier and remark the importance of genetic testing in women with diminished ovarian reserve with apparently normal karyotype for a proper genetic counseling.