At what phase is the cell cycle arrested by an inhibitor? One potential medical treatment to stop cancer cell proliferation employs an inhibitor derived from human umbilical cord stem cells. In this exercise, you will compare two histograms to determine where in the cell cycle the inhibitor blocks the division of cancer cells. In the treated sample, human glioblastoma (brain cancer) cells were grown in tissue culture in the presence of inhibitor-producing umbilical cord stem cells. In contrast, control sample glioblastoma cells were grown in the absence of stem cells. To get a "snapshot" of the phase of the cell cycle each cell was in at the end of 72 hours, the cell samples were treated with a fluorescent chemical that binds to DNA. Next the samples were run through a flow cytometer, an instrument that records the fluorescence level of each cell. Computer software then graphed the number of cells in each sample with a particular fluorescence level.