X-linked lymphoproliferative syndrome (XLP or Duncan disease) is characterized by extreme sensitivity to Epstein-Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal infectious mononucleosis, acquired hypogammaglobulinemia and malignant lymphoma. We have identified a gene, SH2D1A, that is mutated in XLP patients and encodes a novel protein composed of a single SH2 domain. SH2D1A is expressed in many tissues involved in the immune system. The identification of SH2D1A will allow the determination of its mechanism of action as a possible regulator of the EBV-induced immune response.

X-linked hypophosphatemic rickets (HYP) is a dominant disorder characterised by impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules. By positional cloning, we have isolated a candidate gene from the HYP region in Xp22.1. This gene exhibits homology to a family of endopeptidase genes, members of which are involved in the degradation or activation of a variety of peptide hormones. This gene (which we have called PEX) is composed of multiple exons which span at least five cosmids. Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients, which suggest that the PEX gene is involved in the HYP disorder.

The X-lined gene for Norrie disease, which is characterized by blindness, deafness and mental retardation has been cloned recently. This gene has been thought to code for a putative extracellular factor; its predicted amino acid sequence is homologous to the C-terminal domain of diverse extracellular proteins. Sequence pattern searches and three-dimensional modelling now suggest that the Norrie disease protein (NDP) has a tertiary structure similar to that of transforming growth factor beta (TGF beta). Our model identifies NDP as a member of an emerging family of growth factors containing a cystine knot motif, with direct implications for the physiological role of NDP. The model also sheds light on sequence related domains such as the C-terminal domain of mucins and of von Willebrand factor.

DiGeorge syndrome (DGS) is a developmental field defect, characterised by absent/hypoplastic thymus and parathyroid, and conotruncal heart defects, with haploinsufficiency loci at 22q (DGS1) and 10p (DGS2). We performed fluorescence in situ hybridisations (FISH) and polymerase chain reaction (PCR) analyses in 12 patients with 10p deletions, nine of them with features of DGS, and in a familial translocation 10p;14q associated with midline defects. The critical DGS2 region is defined by two DGS patients, and maps within a 1 cM interval including D10S547 and D10S585. The other seven DGS patients are hemizygous for both loci. The breakpoint of the reciprocal translocation 10p;14q maps at a distance of at least 12 cM distal to the critical DGS2 region. Interstitial and terminal deletions described are in the range of 10-50 cM and enable the tentative mapping of loci for ptosis and hearing loss, features which are not part of the DGS clinical spectrum.

For >3 decades, Giemsa banding of metaphase chromosomes has been the standard karyotypic analysis for pre- and postnatal diagnostic applications. However, marker chromosomes or structural abnormalities are often encountered that cannot be deciphered by G-banding alone. Here we describe the use of multiplex-FISH (M-FISH), which allows the visualization of the 22 human autosomes and the 2 sex chromosomes, in 24 different colors. By M-FISH, the euchromatin in marker chromosomes could be readily identified. In cases of structural abnormalities, M-FISH identified translocations and insertions or demonstrated that the rearranged chromosome did not contain DNA material from another chromosome. In these cases, deleted or duplicated regions were discerned either by chromosome-specific multicolor bar codes or by comparative genomic hybridization. In addition, M-FISH was able to identify cryptic abnormalities in patients with a normal G-karyotype. In summary, M-FISH is a reliable tool for diagnostic applications, and results can be obtained in