In situ RT-PCR amplification of CD34 or clonotypic IgH VDJ mRNA in sorted CD19⁺ MM or normal donor B cells. Sorted CD19⁺ MM, CD34⁺19⁺ MM, or normal donor PBMC were placed on slides and using primers to CD34 (top row, CD19⁺ MM PBMC or normal CD19⁺ PBMC) or to patient-specific IgH VDJ (middle row, Pt-1 primers on MM Pt-1 CD34⁺19⁺ PBMC or normal PBMC CD19⁺ B cells; bottom row, Pt-2 primers on MM Pt-2 CD34⁺19⁺ PBMC or normal CD19⁺ PBMC B cells), mRNA was amplified using digoxygenin-labeled nucleotide, followed by visualization with antidigoxygenin antibody in a colorimetric assay. The experiment of the top row used a blue dye, Nitro blue tetrazolium chloride, as substrate while the experiment of the middle and bottom rows used Fast Red as the substrate. Positive cells exhibit intense color deposition. MM CD34⁺19⁺ blood cells are shown in column 1 (left) and CD19⁺ B cells from blood of normal donors in column 2 (right).