Heritable fragile sites on human chromosomes I. Factors affecting expression in lymphocyte culture.

The expression of heritable fragile sites on human chromosomes has been shown to be dependent upon composition of the tissue medium for sites at 2q11, 10q23, 11q13, 16p124, 20p11 and Xq27 or 28 but not for the site at 16q22. Expression of the fragile sites is inhibited by folic acid, thymidine, folinic acid, and probably bromodeoxyuridine, and induced by methrotrexate. In addition, there is a correlation between frequency of expression of the sites and pH of the culture medium for the sites on 2q, 10q and Xq. Possible reasons for these findings are discussed, and a definition and classification of fragile sites is proposed.