Correlation between ErbB1/ErbB4 expression and HB-EGF activity on myeloma cells.ErbB1/ErbB4–positive myeloma cells (A) and ErbB1/ErbB4–negative myeloma cells (B) were cultured for 5 to 7 days in RPMI 1640, 5% FCS, and 5 pg/mL IL-6, without (□) or with (▪) HB-EGF and then pulsed with tritiated thymidine for 12 hours at the end of the culture. Data are expressed as means ± SDs of tritiated thymidine incorporation determined on sixplicate culture well. *Indicates that the mean value is statistically significantly different from that obtained with 5 pg/mL IL-6 alone, using a Student t test (P ≤ .05).