On the frequency of telomeric chromosomal changes induced by culture conditions suitable for fragile X expression.

Under culture conditions suitable for the expression of the fragile site Xq27, "nonspecific" telomeric structural changes similar to the "specific" fra(X) formation occurred apparently on every chromosome arm. Significant differences between individuals seem to exist. The total frequency of nonspecific terminal lesions not located on the long arm of the X chromosome was 0.22 +/- 0.17 per cell in 37 cultures examined. If telomeric lesions on Xq occur in more than 0.7% of the cells from a single culture in males and more than 1.5% of the cells from a single culture in females, then this probably indicates a specific fra(X) expression. Lower percentages may be the result of nonspecific telomeric structural changes in q. These are expected to occur in the normal X as well and may, therefore, give rise to false positive diagnoses in the detection of hemi-, and hetero-, and perhaps also homozygous fra(X) carriers.