PCR amplification of Ig heavy chain gene using the consensus VH and JH primers. PCR products from patients with HBLD (cases no. 1 through 4) showed a diffuse smear, indicating a polyclonal B-cell population (lanes 1 through 4). Lane 5, positive control (a patient with precursor B-cell acute lymphocytic leukemia); lane 6, negative control (placental DNA); lane M, size markers.