Expression of the rare fragile site FRA16B in chromosome band 16q22 was studied in bone marrow (BM) cells and peripheral blood lymphocytes (PBLs) from three unrelated subjects. Although FRA16B was detected only in PBLs from two healthy subjects who had been previously treated for non-Hodgkin's lymphoma (NHL), both cell types displayed FRA16B in a patient with chronic myelomonocytic leukemia (CMMoL). This variability in expression could be related to differences in the proliferative status of the cell populations or to differences in gene activity in the fragile site region.