Gold nanoparticles significantly inhibit proliferation in multiple myeloma cell lines (see figure) in a dose-dependent manner. Gold-treated multiple myeloma cells show cell-cycle arrest in the G1-phase (the phase when chromosomes prepare for replication) by up-regulation of cell-cycle cyclin-dependent kinase inhibitor proteins p21 and p27. Gold nanoparticles do not affect normal peripheral blood mononuclear cells.