Magnesium-dependent hypocalcaemia (HSH), a rare inherited disease, is caused by selective disorders of magnesium absorption. Both X-linked and autosomal recessive modes of inheritance have been reported for HSH; this suggests a genetically heterogeneous condition. A balanced de novo t(X;9)(p22;q12) translocation has been reported in a female manifesting hypomagnesemia with secondary hypocalcemia. In a lymphoblastoid cell line, derived from this patient, the normal X chromosome is preferentially inactivated, suggesting that the patient's phenotype is caused by disruption of an HSH gene in Xp22. In an attempt to define more precisely the position of the X breakpoint, we have constructed a hybrid cell line retaining the der(X)(Xqter-Xp22.2::9q12-9qter) in the absence of the der(9) and the normal X chromosome. Southern blot analysis of this hybrid and in situ hybridization on metaphase chromosomes have localized the breakpoint between DXS16 and the cluster (DXS207, DXS43), in Xp22.2. Thus, if a gene involved in HSH residues at or near the translocation breakpoint, our findings should greatly facilitate its isolation.