The H-2 region of mouse chromosome 17 is known to include one or more genes that affect susceptibility to cortisone-induced cleft palate. We have now studied congenic strains that possess crossovers in the interval between H-2S and H-2D and have observed significant differences in susceptibility among recombinants that had been believed to possess the same H-2 haplotypes. Pregnant mice were injected on days 11 through 14 of gestation with 100 mg of cortisone per kg of body weight. The frequency of cleft palate in B10.A(2R) was significantly greater than in B10.A(1R), despite the fact that both have H-2a/H-2b crossovers in the interval between the S and D loci and have the same alleles at all loci that have been previously characterized. Both B10.BAR5 and B10.BAR12 were significantly more susceptible than B10.A(18R), although these strains also share the same alleles at all loci that have been previously characterized. All three of these strains have H-2b/H-2a recombinant chromosomes, with crossovers in the S/D interval. Genetic linkage between H-2 and the high-susceptibility gene of B10.BAR5 was confirmed by testing H-2 homozygotes derived by intercrossing backcross animals. These data therefore suggest that a gene coding for susceptibility, which we designate Cps-1, maps in the 350-kb interval between H-2S and H-2D, and the congenic strains that we have found to be different have different crossover points within this interval. Alleles at the Cps-1 locus have embryonic effects, but no demonstrable effects on the maternal environment.

Individuals with developmental delay who are found to have only fragile X premutations present an interpretive dilemma. The presence of the premutation could be an unrelated coincidence, or it could be a sign of mosaicism involving a full mutation in other tissues. To investigate three cases of this type, buccal epithelium was collected on cytology brushes for Southern blot analysis. In one notable case, the blood specimen of a boy with developmental delay was found to have a premutation of 0.1 extra kb, which was shown by PCR to be an allele of 60 ± 3 repeats. There was no trace of a full mutation. Mosaicism was investigated as an explanation for his developmental delay, although the condition was confounded by prematurity and other factors. The cheek epithelium DNA was found to contain the premutation, plus a methylated full mutation with expansions of 0.9 and 1.5 extra kb. The three populations were nearly equal in frequency but the 1.5 kb expansion was the most prominent. Regardless of whether this patient has clinical signs of fragile X syndrome, he illustrates that there can be gross tissue‐specific differences in molecular subpopulations in mosaic individuals. Because brain and epithelium are more closely related embryonically than are brain and blood, cryptic full mutations in affected individuals may be evident in epithelial cells while being absent or difficult to detect in blood. This phenomenon may explain some atypical cases of the fragile X phenotype associated with premutations or near‐normal DNA findings. © 1996 Wiley‐Liss, Inc.

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