APM component expression in normal, premalignant, and malignant plasma cells. Control patients (□), MGUS patients (; n = 20), and MM patients (; n = 20) are shown. BMMCs were immunostained for surface expression of CD38 (controls) or both CD38 and CD138 (MGUS and MM), HLA class I antigens, β₂m, and for intracellular expression of individual APM proteins and analyzed by flow cytometry. (A,C,E,G,I) Percentages of CD38⁺ or CD38⁺CD138⁺ cells that stained positively for APM components, expressed as median, interquartile range (box), and range (whiskers). (B,D,F,H,J) Expression levels in positive cells, reported in units of MESF and expressed as median and standard deviation. *P < .05, **P ≤ .005, ***P < .001, Kruskal-Wallis test for variance among the 3 groups.