CD81 expression is upregulated at the time when HSCs (SPKLS) are proliferating in response to 5FU stimulation, a chemotheraputic agent that induces HSC to proliferate. The expression of CD81 is found at a background level in quiescent stages (5FU-Day0 and 5FU-Day11), and is upregulated during proliferation stages (starting 5FU-Day2)