Exclusion mapping of the gene for X-linked neural tube defects in an Icelandic family.

Various polymorphic markers with a random distribution along the X chromosome were used in a linkage analysis performed on a family with apparently X-linked recessive inheritance of neural tube defects (NTD). The lod score values were used to generate an exclusion map of the X chromosome; this showed that the responsible gene was probably not located in the middle part of Xp or in the distal region of Xq. A further refining of these results was achieved by haplotype analysis, which indicated that the gene for X-linked NTD was located either within Xp21.1-pter, distal from the DMD locus, or in the region Xq12-q24 between DXS106 and DXS424. Multipoint linkage analysis revealed that the likelihood for gene location is highest for the region on Xp. The region Xq26-q28, which has syntenic homology with the segment of the murine X chromosome carrying the locus for 'bent tail' (Bn), a mouse model for X-linked NTD, is excluded as the location for the gene underlying X-linked NTD in the present family. Thus, the human homologue of the Bn gene and the present defective gene are not identical, suggesting that more than one gene on the X chromosome plays a role in the development of the neural tube.