Standard curve expression of IL-1β, IL-6, tumor necrosis factor alpha (TNF-α), matrix metalloproteinase 1 (MMP-1) and GAPDH. Standard curves were generated for each cytokine using quantitative PCR on activated peripheral blood mononuclear cells. The graph shows the threshold cycle, C(t), where detectable PCR product was observed, versus the template from a known number of cells. Note that the slopes of each target gene differ, reflecting different amplification efficiency. Unless the slopes are identical, the housekeeping gene cannot be used directly to normalize for RNA content.