Human myeloma cell lines L363 and MM1S were purchased from the American Type Culture Collection and were retro-virally transduced (L363-GFP and MM1S-mCherry) as described previously.25 (link),26 (link) Human MSCs were obtained from the acetabular BM of patients undergoing hip replacement surgery as described previously.23 (link) Primary EPCs were obtained from umbilical cord blood collected from full-term pregnancies as described previously.27 (link) Human BM was obtained from the spina iliaca posterior superior of myeloma patients. The CD138+ cell population was isolated from the mononuclear cells of the myeloma BM by microbeads (Miltenyi Biotec, Bergisch Gladbach, Germany) according to manufacturer’s protocol, and used immediately in the coculture system. Used protocols were approved by the local ethics committee of the University Medical Center Utrecht in accordance with the Declaration of Helsinki; all samples were obtained after written informed consent. Culture conditions can be found in the Materials and methods in the Supplementary material.