Informed choice in fragile X syndrome and its effects on prevalence

We present the effect of case finding, cascade testing, and counselling for fragile X syndrome in a population of 6.5 million over a decade. Carrier females made informed choices that resulted in a 10-fold decrease in the prevalence of affected males in their offspring.

Refinement of the background genetic map of Xq26-q27 and gene localisation for Börjeson-Forssman-Lehmann Syndrome

A detailed map of genetic markers was constructed around the gene for the X-linked mental retardation syndrome of Börjeson-Forssman-Lehmann (BFLS). A multipoint linkage map of framework markers across Xq26-27, based on CEPH families, was integrated with the physical map, based on a YAC contig, to confirm marker order. The remaining genetic markers, which could not be ordered by linkage, were added to create the comprehensive genetic back-ground map, in the order determined by physical mapping, to determine genetic distances between adjacent markers. This background genetic map is applicable to the refinement of the regional localisation for any disease gene mapping to this region. The BFLS gene was localised using this background map in an extended version of the family described by Turner et al. [1989]. The regional localisation for BFLS extends between recombination events at DXS425 and DXS105, an interval of 24.6 cM on the background genetic map. The phenotypic findings commonly seen in the feet of affected males and obligate carrier females may represent a useful clinical indicator of carrier status in potential female carriers in the family. Recombination between DXS425 and DXS105 in a female with such characteristic feet suggests that the distal limit of the regional localisation for the BFLS gene might reasonably be reduced to DXS294 for the purpose of selecting candidate genes, reducing the interval for the BFLS gene to 15.5 cM. Positional candidate genes from the interval between DXS425 and DXS105 include the SOX3 gene, mapped between DXS51(52A) and DXS98(4D-8). SOX3 may have a role in regulating the development of the nervous system. The HMG-box region of this single exon gene was examined by PCR for a deletion and then sequenced. No deviation from normal was observed, excluding mutations in the conserved HMG-box region as the cause of BFLS in this family.

Prevalence of fragile X syndrome

The much-quoted prevalence figure of 1:1,000 males for fragile X syndrome is an overestimate in a mixed ethnic population. A reexamination of the individuals from whom those data were derived using molecular diagnostic techniques demonstrates a more realistic figure of 1:4,000 males.

Synthesis, structure and thermodynamic properties of 8-methylguanine-containing oligonucleotides: Z-DNA under physiological salt conditions

Various oligonucleotides containing 8-methylguanine (m8G) have been synthesized and their structures and thermodynamic properties investigated. Introduction Of M8G into DNA sequences markedly stabilizes the Z conformation under low salt conditions. The hexamer d(CGC[M8G]CG)2 exhibits a CD spectrum characteristic of the Z conformation under physiological salt conditions. The NOE-restrained refinement unequivocally demonstrated that d(CGC[m8G]CG)2 adopts a Z structure with all guanines in the syn conformation. The refined NMR structure is very similar to the Z form crystal structure of d(CGCGCG)2, with a root mean square deviation of 0.6 between the two structures. The contribution of m8G to the stabilization of Z-DNA has been estimated from the mid-point NaCl concentrations for the B-Z transition of various m8G-containing oligomers. The presence of m8G in d(CGC[m8G]CG)2 stabilizes the Z conformation by at least deltaG = -0.8 kcal/mol relative to the unmodified hexamer. The Z conformation was further stabilized by increasing the number of m8Gs incorporated and destabilized by incorporating syn-A or syn-T, found respectively in the (A,T)-containing alternating and non-alternating pyrimidine-purine sequences. The results suggest that the chemically less reactive m8G base is a useful agent for studying molecular interactions of Z-DNA or other DNA structures that incorporate syn-G conformation.

Neomycin, spermine and hexaamminecobalt (III) share common structural motifs in converting B- to A-DNA

The (dG)n.(dC)n-containing 34mer DNA duplex [d(A2G15C15T2)]2 can be effectively converted from the B-DNA to the A-DNA conformation by neomycin, spermine and Co(NH3)6(3+). Conversion is demonstrated by a characteristic red shift in the circular dichroism spectra and dramatic NMR spectral changes in chemical shifts. Additional support comes from the substantially stronger CH6/GH8-H3'NOE intensities of the ligand-DNA complexes than those from the native DNA duplex. Such changes are consistent with a deoxyribose pucker transition from the predominate C2'-endo (S-type) to the C3'-endo (N-type). The changes for all three ligand-DNA complexes are identical, suggesting that those three complex cations share common structural motifs for the B- to A-DNA conversion. The A-DNA structure of the 4:1 complex of Co(NH3)6(3+)/d(ACCCGCGGGT) has been analyzed by NOE-restrained refinement. The structural basis of the transition may be related to the closeness of the two negatively charged sugar-phosphate backbones along the major groove in A-DNA, which can be effectively neutralized by the multivalent positively charged amine functions of these ligands. In addition, ligands like spermine or Co(NH3)6(3+) can adhere to guanine bases in the deep major groove of the double helix, as is evident from the significant direct NOE cross-peaks from the protons of Co(NH3)6(3+) to GH8, GH1 (imino) and CH4 (amino) protons. Our results point to future directions in preparing more potent derivatives of Co(NH3)6(3+) for RNA binding or the induction of A-DNA.

Dissemination of genetic risk information to relatives in the fragile X syndrome: guidelines for genetic counselors

Fragile X Syndrome, which affects 1 in 1,250 males, is the most common inherited condition causing mental retardation. Although carrier detection for the fragile X syndrome utilizing DNA has now been simplified, genetic counseling and the process of informing at-risk family members remains complex. The purpose of this paper is to offer practical guidelines to health professionals providing genetic counseling to fragile X families in order to facilitate the dissemination of genetic risk information to relatives. This paper was developed from a workshop held at the 4th International Fragile X Conference. The guidelines presented here represent a beginning in the development of an approach to informing relatives in fragile X families about genetic risk, and the identification of mechanisms to reduce the burden to families.

Karyotypic evolution in CLL: identification of a new sub-group of patients with deletions of 11q and advanced or progressive disease

Chronic lymphatic leukaemia (CLL) is the most common leukaemia and is characterized by long-term survival. Previous studies have shown that karyotypic abnormalities are relatively stable and that certain abnormalities may be associated with a poor prognosis. In a prospective 5-year study of 45 patients with typical CLL, sequential karyotypic studies were undertaken every 6-12 months. Clonal karyotypic abnormalities were identified in 62% of patients, either at diagnosis or during the study period with 38% (17/45) exhibiting clonal evolution. In patients with no clinical disease progression, 13q abnormalities were most commonly detected compared with 11q deletions in patients with progressive disease. Karyotypic evolution was significantly associated with progressive disease (12/16, 75% vs 5/29, 17%; P < 0.001, chi2). Thus, karyotypic evolution is not uncommon in CLL, is usually associated with disease progression and deletions of 11q are the commonest detected abnormalities.

General overgrowth in the fragile X syndrome: variability in the phenotypic expression of the FMR1 gene mutation

The fragile X syndrome, which often presents in childhood with overgrowth, may in some cases show some diagnostic overlap with classical Sotos syndrome. We describe four fragile X patients with general overgrowth, all of whom are from families with other affected relatives who show the classic Martin-Bell phenotype. Molecular studies of the FMR1 gene in all cases showed the typical full mutation as seen in males affected by the fragile X syndrome. Endocrine studies were unremarkable, except in one case where there were raised levels of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3)

These cases illustrate the clinical variability of the fragile X syndrome and the necessity of performing analysis of the FMR1 gene in mentally retarded patients presenting with general overgrowth.

Influence of counter-ions on the crystal structures of DNA decamers: binding of [Co(NH3)6]3+ and Ba2+ to A-DNA

A-DNA is a stable alternative right-handed double helix that is favored by certain sequences (e.g., (dG)n.(dC)n) or under low humidity conditions. Earlier A-DNA structures of several DNA oligonucleotides and RNA.DNA chimeras have revealed some conformational variation that may be the result of sequence-dependent effects or crystal packing forces. In this study, four crystal structures of three decamer oligonucleotides, d(ACCGGCCGGT), d(ACCCGCGGGT), and r(GC)d(GTATACGC) in two crystal forms (either the P6(1)22 or the P2(1)2(1)2(1) space group) have been analyzed at high resolution to provide the molecular basis of the structural difference in an experimentally consistent manner. The study reveals that molecules crystallized in the same space group have a more similar A-DNA conformation, whereas the same molecule crystallized in different space groups has different (local) conformations. This suggests that even though the local structure is influenced by the crystal packing environments, the DNA molecule adjusts to adopt an overall conformation close to canonical A-DNA. For example, the six independent CpG steps in these four structures have different base-base stacking patterns, with their helical twist angles (omega) ranging from 28 degrees to 37 degrees. Our study further reveals the structural impact of different counter-ions on the A-DNA conformers. [Co(NH3)6]3+ has three unique A-DNA binding modes. One binds at the major groove side of a GpG step at the O6/N7 sites of guanine bases via hydrogen bonds. The other two modes involve the binding of ions to phosphates, either bridging across the narrow major groove or binding between two intra-strand adjacent phosphates. Those interactions may explain the recent spectroscopic and NMR observations that [Co(NH3)6]3+ is effective in inducing the B- to A-DNA transition for DNA with (G)n sequence. Interestingly, Ba2+ binds to the same O6/N7 sites on guanine by direct coordinations.

Protection of ferrets against influenza challenge with a DNA vaccine to the haemagglutinin

Immunization of ferrets with a plasmid DNA expressing influenza virus haemagglutinin (pCMV/H1 DNA) provided complete protection from challenge with the homologous A/PR/8/34 (H1N1) influenza virus. Delivery of DNA-coated gold beads by gene gun to the epidermis was much more efficient than intramuscular delivery of DNA in aqueous solution. The antibody response induced by DNA delivered by gene gun was more cross-reactive than DNA delivered in aqueous solution or after natural infection. This novel approach to vaccination against influenza may afford broader protection against antigenic drift than that provided by natural infection.

Structure and dynamics of the antitumor drugs nogalamycin and disnogalamycin complexed to d(CGTACG)2: comparison of crystal and solution structures

The nuclear magnetic resonance (NMR) solution structures of the 2:1 complexes of nogalamycin-d(CGTACG)2 (Ng-CGTACG) and disnogalamycin-d(CGTACG)2 (DNg-CGTACG) have been determined by a quantitative treatment of two-dimensional nuclear Overhauser effect (2D-NOE) crosspeak intensities. The 1.3 A resolution crystal structure of the 2:1 complex of Ng-CGTACG was used as a starting model for refinement using the procedure, SPEDREF [Robinson and Wang, Biochemistry 31 (1992) 3524-3533], which incorporates full matrix relaxation theory and simulated annealing minimization. The refined solution structures have R-factors of 16.1 and 19.6% between the observed and simulated NOEs for Ng-CGTACG and DNg-CGTACG, respectively. The refined NMR structures retain major features of the crystal structure in which the elongated aglycone chromophore is intercalated between the CpG steps with its nogalose and aminoglucose lying in the minor and major grooves, respectively. The root mean square deviation between the solution and crystal structure for the complexes is 1.01 A (Ng-CGTACG) and 1.20 A (DNg-CGTACG) for the drug, plus the three base pairs surrounding the drug, indicating a very similar local structure at the intercalation site. In the NMR structure, the two G:C Watson-Crick base pairs (C1:G12 and G2:C11) that wrap around the aglycone have large buckles, as do those seen in the crystal structure. There is a 22 degree bend at the T3-A4 step in the refined solution structure. This rearrangement of the solution conformation is likely due to the absence of crystal packing. Specific hydrogen bonds between the drug and G:C bases in both grooves of the helix are preserved in the solution structure. A separate study of the 2:1 complex at low pH showed that the terminal G-C base pairing is destabilized.

Lifetime needle sharing: a predictive analysis

Through understanding predictors of needle sharing, it may be possible to design AIDS prevention interventions more effectively. Data were collected from a sample of 416 patients in two New York City methadone programs in 1990. Questions were asked about needle sharing and about a battery of predictors covering 11 psychosocial domains. Based on factor analysis, these were reduced to seven factors: criminal history, antisocial characteristics, social integration, severity of psychiatric problems, current drug involvement, physical health, and personality disorders. Bivariate analyses showed that criminal involvement, antisocial characteristics, social integration, and age were significantly related to needle sharing. With the seven factors, as well as age, gender, and ethnicity simultaneously examined by means of regression analysis, it was found that criminal involvement, severity of psychiatric problems, and age were all positively associated with needle sharing. Implications for treatment are discussed.

Counselling risk figures for fragile X carrier females of varying band sizes for use in predicting the likelihood of retardation in their offspring

We have derived risk figures for fra(X) syndrome carrier mothers based on their DNA status. Clinical and molecular information was analysed in 200 carrier mothers and their offspring. Individuals were classified as affected by a requirement for special education. Risk figures were calculated using the genotype of the intellectually normal offspring in order to reduce ascertainment bias. Analysis was made on women with differing mutation size to predict the proportion of affected offspring. Using this method the following risk figures were derived: 1. For carrier women with an increase (delta) of 0.06-0.14 Kb, the risk for having an affected son was 29% (1 in 3.5) and 25% for daughters (1 in 4). This predicts an overall 73% chance of a normal child. 2. For delta size 0.15-0.24 Kb, the risk of having an affected son was 46% (1 in 2.2) and 32% for daughters (1 in 3.1), predicting a 61% chance of a normal child. 3. For delta size > 0.24 Kb, normal transmitting male offspring were not seen, i.e., the risk for males was 50% (1 in 2) and for females 32% (1 in 3.1) which predicts a 59% chance of a normal child.

Ultrasound-guided fetal intravascular transfusions for severe erythroblastosis, 1984-1993

The results of the first 10 years' experience in ultrasound-guided fetal intravascular transfusions at the Royal Women's Hospital were reviewed. Since the first transfusion, a variety of techniques have been employed in 78 fetuses, all with severe erythroblastosis. A total of 288 intrauterine transfusions have been attempted with an overall survival rate of 75.6% (59 of 78). The overall survival rate for delivered fetuses improved from 64.3% (18 of 28) in 1984-1987, to 82.0% (41 of 50) in 1988-1993. There was a total of 33 hydropic fetuses, of whom 20 (60.6%) survived, significantly fewer compared with 86.7% (39 of 45) of the nonhydropic fetuses (odds ratio [OR] 0.25, 95% confidence interval [CI] 0.09 to 0.70, p < .01). Fetuses who were sicker at the time of transfusion, as reflected by larger haemoglobin deficits, had lower survival rates, as did those requiring transfusions at earlier gestational ages. When these variables were allowed for, the survival rate significantly improved over time (OR 6.3, 95% CI 1.3 to 30.4, p < 0.05), probably reflecting the increased skill of the ultrasonologists, but the presence of hydrops per se was no longer important. Variations of the technique employed, such as exchange or intraperitoneal transfusion, or different sites for transfusion, were not significantly related to survival.

Solution structure of a GA mismatch DNA sequence, d(CCATGAATGG)2, determined by 2D NMR and structural refinement methods

GA mismatches in DNA have drawn attention because of their special repair mechanisms, stability, and variety of conformations. A symmetric 10-base oligodeoxyribonucleotide duplex, d(CCATGAATGG)2, containing two GA mismatches has been investigated by one- and two-dimensional multinuclear NMR and molecular refinement procedures to ascertain the conformational details of the 5'-pyrimidine-GA-purine-3' sequence. A molecular model established from the NMR results has a B-type right-handed helix with each of the bases retaining the normal anti-glycosidic torsional angles. Type I mismatched base pairs have GNH2-AN7 and GN3-ANH2 (edge-to-edge) hydrogen bonds, while type II base pairs have GN1H-AN1 and GO6-ANH2 (face-to-face) bonds. The conformation at the GA mismatch site has type I GA base pairs and an unusual cross-strand stacking of the adjacent G5 and A6 bases, which causes significant overwinding of the helix at the mismatch site. Unusual shifts of the 31P resonances suggest that the phosphate linkage between G5 and A6 is no longer in the low-energy BI conformation. One-dimensional imino and phosphorus NMR studies were carried out on a number of DNA sequences containing adjacent 5'-GA-3' mismatched base pairs to investigate the sequence dependence of the conformations and base-pairing types. Type I and type II conformations have very different imino proton and 31P NMR spectral patterns that can be used to classify any sequence with adjacent GA mismatches by base-pairing and conformational type. The NMR results indicate that the conformation selected is dictated completely by the flanking sequence: 5'-pyrimidine-GA-purine-3' sequences adopt the type I conformation, while 5'-purine-GA-pyrimidine-3' sequences have the type II conformation.

Neonatal alloimmune thrombocytopenia. A case report and a review of the literature

A 32-year-old woman in her third pregnancy underwent fetal blood sampling because of a previous child with neonatal thrombocytopenia. At 33 weeks' gestation, fetal thrombocytopenia was diagnosed. Treatment was instituted antenatally with serial fetal platelet transfusions and corticosteroid therapy. Delivery was by Caesarean section at 37 weeks' gestation. Neonatal treatment included further platelet transfusion and immunoglobulin infusion. Recovery of the neonate was complete on discharge from hospital 10 days after birth. The aetiology, diagnosis, clinical presentations and therapeutic options in cases of alloimmune thrombocytopenia are discussed.

Molecular structure of cyclic diguanylic acid at 1 A resolution of two crystal forms: self-association, interactions with metal ion/planar dyes and modeling studies

Cyclic ribodiguanylic acid, c-(GpGp), is the endogenous effector regulator of cellulose synthase. Its three dimensional structure from two different crystal forms (tetragonal and trigonal) has been determined by x-ray diffraction analysis at 1 A resolution. Both structures were solved by direct methods and refined by block-matrix least squares refinement to R-factors of 0.112 (tetragonal) and 0.119 (trigonal). In both crystal forms, two independent c-(GpGp) molecules associate with each other to form a self-intercalated dimer. All four c-(GpGp) molecules have very similar backbone conformation. The riboses are in the C3'-endo pucker with pseudorotation angles ranging from -7.2 degrees to 16.5 degrees and the bases have anti glycosyl chi angles (-175.5 degrees to 179.7 degrees). In the tetragonal form, a hydrated cobalt ion is found to coordinate to two N7 atoms of adjacent guanines, forcing these two guanines to destack with a large dihedral angle (33 degrees). This metal coordination mechanism has been noted previously in other Pt- or Co-GMP complexes and may be relevant to the binding of the anticancer drug cisplatin to a GpG sequence in DNA. A model of the adduct between cisplatin and a d(CAATGGATTG) duplex has been constructed in which the induced bending of the DNA helix at the Pt crosslinking site is 33 degrees, consistent with earlier electrophoretic analyses. Moreover, c-(GpGp) exhibits unusual spectral properties not seen in other cyclic dinucleotides. It interacts with planar organic intercalator molecules in ways similar to double helical DNA. We propose a cage-like model consisting of a tetrameric c-(GpGp) aggregate in which a large cavity (host molecule) is generated to afford a binding site for certain planar intercalators (guests molecules). The aggregate likely uses a hydrogen bonding scheme the same as that found in the G-quartet molecules, e.g., telomere DNA. The conformation of c-(GpGp) also suggests that certain nearest-neighbor intercalators may be synthesized on the basis of its unique molecular framework. Modeling studies have been carried out to test this hypothesis.

Structural effects of the C2-methylhypoxanthine:cytosine base pair in B-DNA: A combined NMR and X-ray diffraction study of d(CGC[m2I]AATTCGCG)

C2-Methylhypoxanthine (m2I) is a synthetic analog of guanine with the N2-amino group replaced by a methyl group. We have studied the structural consequence of the m2I incorporation in DNA by a combination of X-ray crystallographic, NMR, and enzymatic analyses. The crystal structure of d(CGC[m2I]AATTCGCG) has been solved and refined to an R factor of 20.7% at 2.25-A resolution. In the DNA duplex, the two independent m2I:C base pairs maintain the Watson-Crick scheme. While the C2-methyl group of m2I is in van der Waals contact with the O2 of the base-paired cytosine, it only causes the base pair to have slightly higher propeller twist and buckle angles. Its solution structure was analyzed by the NMR refinement procedure SPEDREF [Robinson, H., & Wang, A. H.-J. (1992) Biochemistry 31, 3524-3533] using 2D nuclear Overhauser effect data. Two starting models, a relaxed fiber model and an X-ray model, were subjected to the NOE-constrained refinement using 1518 NOE cross-peak integrals to arrive at the final models with (NOE) R factors of 13.8% and 14.3%, respectively. The RMSD between the two refined models (all atoms included) is 1.23 A, which presently seems to be near the limit of convergence of NOE-based refinement. The local structures of the two models are in better agreement as measured by the RMSD of the dinucleotide steps, falling in the range 0.54-0.98 A. Both refined solution structures confirm that the m2I dodecamer structure is of the B-DNA type with a narrow minor groove at the AT region, as observed in the crystal. However, significant differences exist between the crystal and solution structures in parameters such as pseudorotation angles, propeller twist angles, etc. The solution structure tends to have a more uniform backbone conformation, an observation consistent with that concluded from the laser Raman study of d(CGCAAATTTGCG) [Benevides, J. M., Wang, A. H.-J., van der Marel, G. A., van Boom, J. H., & Thomas, G. J., J. (1988) Biochemistry 27, 931-938]. Three related dodecamers, d(CGCGAATTCGCG), d(CGC[m2I]AATTCGCG), and d(CGC[e6G]AATTCGCG), were tested as substrates for the restriction endonuclease EcoRI. The m2I dodecamer was active, but the e6G dodecamer was not. Our results illustrate the complementarity in terms of the structural information provided by the two methods, X-ray diffraction and NMR.

Silicon lithium niobate electro-optic waveguide modulator structures in the parallel-plate configuration

Waveguide modulators incorporating paralle-plate electrodes are investigated by computer modeling. Metal-electrode structures differ from severe optical losses that are due to surface plasmons. Siliconelectrode structures exhibit lower losses. Bandwidth is limited by the resistivity and the proximity of the electrodes. Doping the silicon improves conductivity but incireases optical absorption. Device optimization involves a trade-off between bandwidth and optical loss. Devices are fabricated by the use of substrates of silicon on sapphire, with rf sputtered lithium niobate films and plasma-enhanced chemical vapor-deposited hydrogenated amorphous silicon for the upper electrodes. The electro-optic coefficient of these lithium niobate films is ~50% of the value for bulk material. The results indicate the possibility of using these devices for combining silicon integrated circuits with waveguide modulators on a common substrate.

5'-CGA sequence is a strong motif for homo base-paired parallel-stranded DNA duplex as revealed by NMR analysis

The structure of the non-self-complementary DNA heptamer d(CGACGAC) at low pH has been determined by the quantitative NMR refinement procedure designated SPEDREF (SPEctral-Driven REFinement). Acid-base titration of the molecule indicated a prominent n = 2 pKa near 6.8. In the pH range up to 6.0, the heptamer forms a remarkably stable double helix, which was conclusively shown to be an unusual homobase-paired parallel-stranded double helix (termed II-DNA). In this II-DNA helix, the 5'-CGA trinucleotide is the structural motif that accounts for the stability, with the C+-C hemiprotonated base pair (in which C+ is N3-protonated cytosine) providing for the alignment site and the unusual interstrand G-A base stack in the GpA step furnishing the additional stabilizing forces. The exchangeable proton data from two-dimensional nuclear Overhauser effect spectroscopy are in total agreement with the refined structure. We conclude that the 5'-CGA or other related sequences (e.g., 5'-CCGA) are powerful motifs in promoting the II-DNA or II-RNA conformations that may play certain biological functions.

Two adjacent residues in staphylococcal enterotoxins A and E determine T cell receptor V beta specificity

The T cell receptor (TCR) V beta-determining region of two bacterial superantigens, staphylococcal enterotoxin A (SEA) and SEE, has been mapped to the COOH-terminal region of SEA and SEE using a panel of recombinant SEA/SEE hybrids. Total TCR V beta mRNA enrichment in human peripheral blood T cell cultures was determined by a novel single-tube amplification technique using a redundant V beta-specific primer. SEA routinely enriched mRNA coding for hV beta 1.1, 5.3, 6.3, 6.4, 6.9, 7.3, 7.4, and 9.1, while SEE, which is 83% homologous to SEA, enriched hV beta 5.1, 6.3, 6.4, 6.9, and 8.1 mRNA. Exchanging residues 206 and 207 was sufficient to convert in toto the TCR V beta response of human peripheral T lymphocytes. In addition, an SEA-reactive murine T cell line, SO3 (mV beta 17), unresponsive to wild-type SEE responded to SEE-S206N207, while an SEE-specific human T cell line, Jurkat (hV beta 8.1), unresponsive to SEA was stimulated strongly by SEA-P206D207. Exchanging all other regions of SEA and SEE except residues 206 and 207 did little to change the V beta response. Thus, the V beta binding region appears to be a stable, discrete domain localized within the COOH-terminal region that is largely unaffected by the considerable amino acid variability between SEA and SEE. This region may interact directly with TCR V beta.