Membrane PR3 expression is restricted to cells with NB1 protein. Cells sorted for mPR3 (n = 5, A) and mNB1 (n = 3, B) expression (purity, 90% ± 5%) were subjected to immunoblot analysis for PR3 and NB1. Corresponding optical densitometry showed no significant difference in whole-cell PR3 content in the membrane-positive and -negative subpopulations. However, NB1 was restricted to mPR3- and mNB1-positive cells (*P < .05). Error bars indicate ± SEM.