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28S ribosomal RNA (rRNA) (2-4), suggesting that the absence of the human rRNA may be related to the loss of human chromosomes. It is not due to the absence of the human acrocentric chromosomes, which carry the structural genes for rRNA (5, 6), because Marshall et al. (4) found no human 28S rRNA in a large series of mousehuman hybrids that contained 2 to 11 human acrocentrics.The chromosome regions that carry the rRNA genes have been identified as the nucleolus organizer regions (NORs) (7), and these regions can be stained preferentially by the Ag-AS silver staining method (8). In human diploid cell cultures the Ag-AS method stains the short arm regions of most of the acrocentric chromosomes (9, 10). The NORs of the same human acrocentric chromosomes are not stained in a mouse-human hybrid that has lost some human chromosomes (11). There is no evidence to suggest that rRNA genes are deleted from the human acrocentrics in hybrid cells. Therefore the absence.of Ag-AS stain suggests that this method detects only chromosome regions that functioned as nucleolus organizers in the preceding interphase, and, by implication, produced rRNA.Somatic cell hybrids between either mouse peritoneal macrophages or mouse cells obtained from a teratocarcinoma and HT-1080 human fibrosarcoma cells retain human chromosomes and lose mouse chromosomes (12). If preferential chromosome elimination is closely correlated with preferential suppression of nucleolus organizer activity, these hybrids should express only human nucleolus organizer activity. This appears to be the case. METHODS BALB/c mouse peritoneal macrophages were obtained according to a modification of the procedure described by Cohn and Benson (13) and were fused with HT-1080-6TG human fibrosarcoma cells deficient in hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) (12) in the presence of f3-propiolactone-inactivated Sendai virus at pH 8.0 (14). The fused cultures were maintained in hypoxanthine-aminopterin-thymidine (HAT) selective medium (15). Large colonies of hybrid cells were visible 3-4 weeks after fusion. The colonies were picked and, subsequently, grown. OTT6050 mouse teratocarcinoma cells were obtained from a solid teratocarcinoma of a strain 129 mouse (16,17) by cutting in small fragments in trypsin/EDTA, resuspending in Eagle's minimal essential medium (MEM), and filtering through sterile gauze. The teratocarcinoma cells were fused with HT-1080-6TG cells in the presence of f.-propiolactone-inactivated Sendai virus. Hybrid colonies were selected in hypoxanthine-aminopterin-thymidine medium.Hybrid cells were maintained in Eagle's medium supplemented with 10% fetal calf serum. Mitotic cells were shaken from the culture flasks and transferred to a centrifuge tube containing 0.1 ml of colcemid (10 Ag/ml) for every 10 ml of medium and the tubes were centrifuged immediately at 800 rpm in an IEC clinical centrifuge for 7 min. The cell pellet was resuspended in 75 mM KCI. After 10 min the suspension was centrifuged and the cells were fixed for an hour in freshly prepa...

Restriction endonucleases cut and partially removed DNA throughout fixed air-dried human metaphase chromosomes. Some enzymes produced a G-banding pattern; some revealed the presence of multiple chromosome-specific classes of highly repetitive DNA in C-band heterochromatin. Enzymes that produced the informative C-band patterns had recognition sequences that were four or five, but not six, base pairs long and did not contain a cytosine-guanine doublet. In both rat and human chromosomes, regions containing amplified ribosomal RNA genes were specifically removed by the restriction endonuclease Msp I.

Clone p82H is a human DNA sequence which hybridises in situ exclusively to the centromeric regions of all human chromosomes. It is composed of approximately 14 tandemly repeated variants of a basic 172 bp sequence, and is related to the alphoid family. The organisation of the family of cross-hybridising sequences, detected by the clone p82H, is described both in the human genome and on certain chromosomes, and its relationship to known sequence families is discussed.

Since some patients with Ullrich-Turner syndrome (UTS) have mental retardation, we reviewed our experience to look for a high-risk subgroup. Among 190 UTS and gonadal dysgenesis patients with X chromosome abnormalities, 12 had mental retardation. All of the six (100%) with a small ring X were educable (EMI) or trainable mentally impaired (TMI) with more severe delay than expected in UTS. Among the 184 with other X abnormalities, only 6 had similar delays (2 from postnatal catastrophes), for a frequency of 3.3% mental retardation among those without a small ring X; only 2.2% of these had unexplained mental retardation. Polymerase chain reaction studies showed no Y-derived material in the 2 patients who were evaluated, and in situ hybridization confirmed X origin of the ring in the 6 subjects who were evaluated. We describe the phenotype of the 6 individuals with a small ring X, and an additional 2 patients with a small ring X who were identified outside the survey. The subjects with a small ring X comprised a clinically distinct subgroup which had EMI/TMI and shorter stature than expected in UTS. Seizures and a head circumference less than 10th centile were observed in half of the patients with a small ring X, and strabismus, epicanthus, and single palmar creases were present in more than half. A "triangular" face in childhood, pigmentary dysplasia, sacral dimple, and heart defects were also common. Neck webbing appeared to be less frequent than in 45,X. We hypothesize that the high risk of mental retardation in this form of the UTS results from lack of lyonization of the ring X due to loss of the X inactivation center. Excluding those with a small ring X, mental retardation is not significantly increased in patients with UTS.