MM PBMC were stained with HPCA-1 or 8G12/goat anti-mouse-PE, and sorted for positive cells, defined as those with staining above the isotype matched control staining. On reanalysis, >95% of sorted cells had CD34 staining. Sorted cells, either total CD34⁺ or CD34⁺ PBMC falling within the small lymphocyte scatter gates, were placed on slides and analyzed using in situ RT-PCR with the indicated primer pairs as described methods. IgH VDJ mRNA was measured using consensus primers. At least 200 cells were counted per slide. Values are the mean ± SD. Only 2 patients gave sufficient small CD34⁺ PBMC to test for CD19 mRNA. The lack of CD34 mRNA in some cells with phenotypic expression of CD34 may indicate downregulation of CD34 mRNA synthesis by these cells, perhaps reflecting the apparent hierarchical character of CD19⁺ PBMC in myeloma.39 Preliminary tests using mobilized stem cell collections suggest that for mobilized CD34⁺ cells, essentially all express CD34 mRNA.

P < .0001 compared with line 1.