Contributions of genes from contaminating cell populations. (A) Closer inspection of some of the genes within the gene cluster overexpressed in normal B cells, CLL, and WM revealed some interesting differences. In particular, the concurrent overexpression of NPTT (tdt), MME (CD10), and VPREB1 in the WM samples suggested likely contamination with pre-B cells. In addition, ZAP70 and CCNB1 (as a representative gene from the proliferation cluster overexpressed in a subset of patients with WM) overexpression is tightly associated with samples overexpressing NPTT (tdt), MME (CD10), and VPREB1. (B) To verify that tdt expression was originating from contaminating pre-B cells, immunostaining for tdt was performed on BM biopsies from patients with WM with and without the pre-B-cell contamination signature. This figure is representative for samples with the contamination signature. Malignant cells (CD20⁺) formed intramedullary clusters (marked by arrows; left panel, hematoxylin and eosin staining; middle panel, CD20 staining). In contrast, nuclear tdt staining was seen in scattered interstitial cells that represented pre-B cells (right panel). In samples without the contamination signature, no tdt⁺ cells were seen. All images were acquired using a 40×/0.8 numeric aperture objective lens. The microscope used is a Zeiss Axioskop (Carl Zeiss Microimaging, Thornwood, NY). Images were captured by the Olympus DP70 CCD camera using DP controller image capture software (Olympus, Center Valley, PA).