RBCs from patients with PV show greater expression of Lu/BCAM. Using specific antibodies and flow cytometry, we measured Lu/BCAM expression on RBCs from 16 healthy volunteers and 23 patients with PV; Lu/BCAM showed higher expression in patients with PV (**P < .01). The unbroken line within the box represents the median; the dotted line represents the mean. The vertical lines extending beyond the boxes indicate the 25% and 75% percentiles, while the horizontal bars outside the boxes represent the 10% and 90% percentiles. ○ indicates the values outside this range.