Fragile sites are chemically induced nonstaining gaps in chromosomes. Different fragile sites vary in frequency in the population and in the chemistry of their induction. DNA sequences encompassing and including the rare, autosomal, folate-sensitive fragile site, FRA16A, were isolated by positional cloning. The molecular basis of FRA16A was found to be expansion of a normally polymorphic p(CCG)n repeat. This repeat was adjacent to a CpG island that was methylated in fragile site-expressing individuals. The FRA16A locus in individuals who do not express the fragile site is not a site of DNA methylation (imprinting), which suggests that the methylation associated with fragile sites may be a consequence and not a cause of their genesis.

The breakpoints of seven interstitial de-letions of the long arm of chromosome 16 and two ring chromosomes of this chromosome were mapped by in situ hybridisation or by analysis of mouse/ human somatic cell hybrids containing the deleted chromosome 16. Use of a high resolution cytogenetic based physical map of chromosome 16 enabled breakpoints to be assigned to an average resolution of at least [1][2][3][4][5][6] Mb.In general, interstitial deletions involving q12 or q22.1 have broadly similar phenotypes though there are differences in specific abnormalities. Deletions involving regions more distal, from 16q22.1 to 16q24.1, were associated with relatively mild dysmorphism. One region of the long arm, q24.2 to q24.3, was not involved in any deletion, either in this study or in any previous report. Presumably, monosomy for this region is lethal. In contrast, patients with deletions of 16q21 have a normal phenotype. These results are consistent with the proposed distribution of genes, frequent in telomeric Giesma light band regions but infrequent in G positive bands. (_J Med Genet 1993;30:828-32 The breakpoints of IDI and 1D3-ID5 were mapped by Southern blot or PCR analysis of mouse/human somatic cell hybrids containing the deleted chromosome.'7 The presence (+) or absence (-) of a DNA marker on the chromosome 16 with a deletion was determined by FISH except in cases indicated by an asterisk where this was determined by inheritance of polymorphic (AC)n microsatellite markers. The order of DNA markers is from centromere to qter. showed head circumference and height at the 25th centile for age and weight at the 3rd centile for age. She had a flat occiput and right epicanthic fold but otherwise was not dysmorphic. Re-examination at 71 years showed a short, microcephalic girl with minor dysmorphism consisting of recessed hairline, slightly prominent broad forehead, supernumerary nipple (present in her normal mother), short and broad big toes, and small nails on the fifth toes. She was retarded, not toilet trained, and had general learning difficulties. Patient ID7. This female was delivered spontaneously at 37 weeks' gestation. Birth weight was 3170 g (50th to 75th centile),length 48 cm (50th centile), and head circumference 34 cm (75th centile). During the first few months there was normal feeding and development. At 8 weeks, clinical examination showed trembling, hypertonia, narrow palpebral fissures with antimongoloid slant, broad nasal root, flat forehead, an extremely large anterior fontanelle, long philtrum, high arched palate, and evidence of micrognathia. The two lower incisors were present at birth (both lost spontaneously at 4 weeks), the ears were low set and dysmorphic, there were flexion defects of the middle fingers, and widely spaced nipples. A photograph of the patient is presented in fig 1. DNA markersDeletions and rings

Three patients with different marker chromosomes were screened by in situ hybridisation using biotinylated probes to chromosome specific pericentric repeats to determine the chromosomal origin of the marker. Each marker had a different origin, with one from each of chromosomes 1, 9, and 16. This is the first time that autosomal marker chromosomes consisting of a small ring have been shown to be derived from the pericentric heterochromatin of metacentric and submetacentric chromosomes. Evidence suggests that such markers are not associated with any significant risk of phenotypic abnormalities, but additional cases need to be studied.Marker chromosomes occur in humans with a frequency of approximately 15 per 1000 with 40% being familial. ' These markers vary in both size and structure. A de novo marker that is small, has an absence of obvious euchromatin, and has distamycin/ DAPI positive material seems to be associated with a low risk of fetal anomalies.' However, studies using conventional cytogenetic staining procedures generally cannot identify the chromosomal origin of marker chromosomes. Among the smaller marker chromosomes an exception is the inv dup(15) where the presence of distamycin/DAPI positive and NOR staining material confirms the origin.2 3

Here we report on a family with an inherited rearrangement of chromosome 8q, dir ins(8)(q24.11q13.3q21.13). Individuals with the chromosome abnormality, which does not appear to be associated with deletion of chromosome material, have manifestations of both tricho-rhino-phalangeal syndrome (TRPS) and branchio-oto syndrome (BOS). TRPS has been linked previously to deletions involving 8q24.11----q24.13, but none of the described patients with deletions in this part of 8q have had characteristics of the BOS. The presence of a breakpoint in 8q24.11 without apparent chromosome deletion in the family described suggests that TRPS maps to this band of 8q. Further, it is suggested that BOS maps to either 8q13.3 or 8q21.13.

TNFR1 and TNFR2, the genes encoding the two forms of the human tumor necrosis factor receptor, were localized to normal human chromosomes by in situ hybridization and Southern blot analysis of a series of human × mouse hybrid cell lines. TNFR1 maps to 12p13 and TNFR2 maps to 1p36.

To define the PKD1 locus further, the gene involved in the most frequent form of adult polycystic kidney disease, probes from 16 polymorphic loci were mapped on 16pI3.1-pter with the combined use of cell lines containing rearranged chromosomes and family studies. Five breakpoints in the distal part of 16p arbitrarily subdivided the loci into five groups. By analysing 58 recombination events among 259 informative meioses in 12 large families with PKD, we were able to construct a linkage map for the distal part of 16p. The order of the markers obtained with chromosomal rearrangements was confirmed by the family studies. The D16S85 locus near a globin, D16S21, and D16S83 map distal, ortelomeric, toPKDI. The polymorphic red cell enzyme phosphoglycolate phosphatase (PGP), D16S84, D16S259, and D16S246 showed no recombination with PKDI. The remaining nine RFLPs all map proximal to the PKD1 gene. By cosmid walking, additional RFLPs were detected at the D16S21 locus. A single intrahaplotype recombination observed defines the orientation of D16S21 relative to PKDL.The new polymorphisms are valuable for presymptomatic and prenatal diagnosis of PKD1. Furthermore, our map is both a good starting point for the physical map of 16p and a useful tool for the isolation of the PKD1 gene.

The fragile X syndrome is the most common cause of familial mental retardation. Genetic counseling and gene isolation are hampered by a lack of DNA markers close to the disease locus. Two somatic cell hybrids that each contain a human X chromosome with a breakpoint close to the fragile X locus have been characterized. A new DNA marker (DXS296) lies between the chromosome breakpoints and is the closest marker to the fragile X locus yet reported. The Hunter syndrome gene, which causes iduronate sulfatase deficiency, is located at the X chromosome breakpoint that is distal to this new marker, thus localizing the Hunter gene distal to the fragile X locus.

The association of small accessory marker chromosomes in man with specific abnormalities has been difficult to define owing to variations in the chromosome origin and the size of the markers. In a patient with typical Turner phenotype and a 45,X/46,X, +mar karyotype the marker was shown to be a small portion ofthe long arm of the X chromosome which included the centromere and XIST, a candidate gene for the X inactivation centre. Therefore the lack of any additional abnormalities was attributed to inactivation of the portion of the X chromosome in the marker. In a patient with a 47,XY, +mar karyotype the mar was a small ring X chromosome which did not contain the XIST gene. For both markers the short arm breakpoints were localised between UBEl and DXS423E. The congenital abnormalities of the male patient were attributed to the lack ofX inactivation ofthe small ring and therefore disomic expression of normal genes possessed by the marker. (JMed Genet 1995;32:113-116) The chromosomal origins of small accessory marker chromosomes may be identified by fluorescence in situ hybridisation (FISH) using chromosome specific pericentric repeat probes. Markers 5 Markers which are derived from the X chromosome and contain the X inactivation centre would be expected to be late replicating (inactive), and therefore be of little clinical significance. In contrast, markers lacking the X inactivation centre would not be inactivated and can therefore contain active genes. For such markers, it would be expected that the extra copies of the normal genes, resulting in two active copies rather than a single active copy, will be sufficient to result in an abnormal phenotype.X inactivation may be determined cytogenetically by study of replication timing during the cell cycle. Using this approach it is difficult to assess the status of very small segments of chromosome, such as a marker. XIST is considered to be candidate gene for X inactivation since it is only expressed on the inactive X chromosome and maps to the region of the X inactivation centre.'2-14 In this report two patients with small ring chromosomes were studied. These rings were identified as X derived and the presence ofthe inactivation centre was determined by FISH using a probe specific for the XIST gene. Materials and methods MOLECULAR CYTOGENETICSThe markers were initially recognised as r(X)s by in situ hybridisation of biotin labelled pericentric repeat probes to metaphase spreads.