In vitro bioluminescence of MDA-MB-231 sublines D3, D3H1 and D3H2LN. Cells from each subline were serially diluted in duplicate wells from 4000 to 8 cells/well. Luciferin substrate was added to each well and the plate was imaged to obtain photons/s per cell. Wells with media (no cells) or cells alone (no luciferin) were included as controls. The range of bioluminescence for each subline represents data collected from two to three experiments.