Evaluation of MMPC markers with respect to robustness. Three candidate markers (CD319, CD307e, and CD269) allowed identification of MMPCs, whereas 4 markers (CD208, GPRC5D, ITGA8, and FKBP11) could not be detected. (A) Protein levels on MMPCs expressed as MFI relative to other cells in samples that had been stored for 6 to 24 hours at 8°C or vital-frozen prior to analysis. As expected, CD138 could be detected at variable levels in fresh samples, but showed considerably lower expression in stored/delayed and frozen samples. By contrast, CD319 and CD269 showed remarkably stable expression (relative MFI approximately 10-100 and approximately 10, respectively) and enabled the identification of a discrete, marker-positive population. Cells isolated from these populations by FACS were confirmed to be MMPCs by costaining with CD138 in fresh samples, clonal excess analysis, light microscopy, and DNA copy number microarray (supplemental Figure 1). CD307e showed more varied expression. (B) Representative example showing that both CD138 and CD319 are present on MMPCs in fresh samples. However, when the same sample was analyzed the following day or after vital-freezing, CD138 was lost, but CD319 was retained. The figure also illustrates how MMPCs (blue events) were discriminated from other cells (green events). (C) Time-series data, in which the same samples were stained/analyzed at varying time points, were between 0 and 40 hours of storage at 8°C.