Fragile sites at 4q23 and 7q11.23 unique to bone marrow cells.

Fragile sites in chromosome bands 4q23 and 7q11.23 were discovered in bone marrow cells. Expression of these fragile sites was induced by treatment of the cells sequentially with 10(-7) M methotrexate and then 10(-5) M thymidine. No expression was observed in bone marrow cells without treatment. The fragile sites at 4q23 and 7q11.23 were seen individually, together, and in the homozygous state in a total of 20 bone marrow samples. The bone marrow karyotypes were normal in all cases. Expression of these common fragile sites at 4q23 and 7q11.23 could not be induced in blood lymphocytes from two subjects using methotrexate and thymidine, or by any other means including the addition of bromodeoxyuridine and fluoro-deoxyuridine or growth in folate-deficient medium. Because the fragile sites at 4q23 and 7q11.23 have never been observed in lymphocytes treated with methotrexate and thymidine for high-resolution chromosome analysis, it appears that these fragile sites are not expressed under these conditions in T lymphocytes. We propose that the differential expression of these fragile sites in bone marrow cells reflects genes active in bone marrow cells but not in blood lymphocytes.