Expression of the ErbB receptors and HB-EGF in purified primary myeloma cells and in the tumor environment. (A) ErbB1, ErbB2, ErbB3, ErbB4, and HB-EGF expression was assayed by RT-PCR on primary myeloma cells. These cells were purified (> 95%) from 9 patients (5 with intramedullar myeloma and 4 with PCL). The A431 epidermoid carcinoma cell line was used as a control. (B) HB-EGF gene expression was analyzed by RT-PCR in unselected mononuclear cells from 8 patients with MM. (C) HB-EGF gene expression was analyzed by RT-PCR in CD14 purified monocytes (> 98%) from 3 patients and in stromal cells derived from 5 patients. Amplification of β actin shows the equivalence of the cDNA loading and amplification. Results are of one experiment representative of 3.