The distamycin A-sensitive fragile site fra(16)(q22) is a precisely localized chromosomal marker. When expressed at metaphase, it visibly separates the chromosome material on either side of the fragile site. Using a cDNA probe encoding both the alpha and beta haptoglobin chains, the haptoglobin loci were found by in situ hybridization to be distal to fra(16)(q22).