Opitz syndrome (OS) is an inherited disorder characterized by midline defects including hypertelorism, hypospadias, lip-palate-laryngotracheal clefts and imperforate anus. We have identified a new gene on Xp22, MiD1 (Midline 1), which is disrupted in an OS patient carrying an X-chromosome inversion and is also mutated in several OS families.MIDI encodes a member of the B-box family of proteins, which contain protein-protein interaction domains, including a RING finger, and are implicated in fundamental processes such as body axis patterning and control of cell proliferation. The association of MIDI with OS suggests an important role for this gene in midline development

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Positional cloning has previously resulted in the identification of a gene which is disrupted by deletions in patients with the classic choroideremia (CHM) phenotype. More subtle mutations had been identified in 4 exons of the 3' portion but not elsewhere in the CHM gene. We have now isolated and characterized the complete open reading frame of the CHM gene and determined its exon-intron structure. The CHM gene encodes a protein of 653 amino acids, which is highly homologous to the mouse and rat CHM proteins, and, to a slightly lesser extent, to the human CHM-like (CHML) protein. The open reading frame (ORF) of the human CHM gene consists of 15 exons, spanning at least 150 kb of Xq21.2, and it is possible that there is an additional exon corresponding to the 5' non-coding region of the gene. Cloning of the 5' end of the CHM gene and the elucidation of its intron-exon structure enabled us to localize the X-chromosomal breakpoint in a CHM female with an X;7 translocation between exons 3 and 4.

In several families with non-specific X-linked mental retardation (XLMR) linkage analyses have assigned the underlying gene defect to the pericentromeric region of the X chromosome, but none of these genes have been isolated so far. Here, we report on the cloning and characterization of a novel gene, DXS6673E, that maps to Xq13.1, is subject to X-inactivation and is disrupted in the 5' untranslated region by a balanced X;13 translocation in a mentally retarded female. The DXS6673E gene is highly conserved among vertebrates and its expression is most abundant in brain. It encodes a hydrophilic protein of 1358 amino acids (aa) that does not show sequence homology to other known proteins. A segment of this protein consisting of neutral and hydrophobic aa with a proline residue in every second position may represent a transmembrane domain. Almost complete sequence identity was found between the 3' end of the DXS6673E gene and two expressed sequence tags (ESTs) and between the 5' end of the DXS6673E gene and a third EST. Moreover, weaker sequence similarity was observed between coding regions and two other ESTs.

Two cases of interstitial deletion of chromosome 15 with similar clinical features are presented. In one case, assay of hexosaminidase A enabled us to confirm that the structural gene is located between 15q22 and 15q25 and that it is included in the deletion.