Relative basal and stimulated levels of tristetraprolin (TTP), ZFP36L1, and ZFP36L2 transcripts in cultured human monocytes. The real-time polymerase chain reaction (PCR) values were normalized to the geometric mean of four internal transcript controls and against PBS-treated cells at corresponding time points, then converted to 2-ΔΔCtto obtain expression level values for the TTP, ZFP36L1, and ZFP36L2 transcripts, shown in (a) as means ± s.e.m. from cultures derived from five individual subjects. These expression levels from the three transcripts can be compared with each other within subjects, because the primer/probe sets for each gene yielded equivalent PCR amplification efficiencies and fluorescence intensities, and the data were normalized to control for plate-to-plate variations. The basal levels are similar for each of the three genes in resting monocytes, shown as 0 hours after lipopolysaccharide (LPS) in (a) and t = 0 in (b). LPS treatment for 1 hour stimulated the expression of all three genes; however, after 1 hour of exposure to LPS, TTP levels were increased most markedly (a and b, right panel). In unstimulated cells, mean levels of all three transcripts representing TTP 'equivalents' were approximately equal (b, left panel). However, after 1 hour of stimulation with LPS, TTP represented about 69% of TTP equivalents, compared with 21% for ZFP36L1 and 10% for ZFP36L2.