Malignant plasma cells secrete functional BDNF. Media conditioned by HMCL JJN3 triggers neurite outgrowth by TrkB-expressing PC12 cells (PC12-TrkB) (A) but not by parental PC12 cells, which lack TrkB (B) and not by PC12-TrkB in the presence of neutralizing anti-BDNF antibody (C).