Analysis of sampling error. Intrasynovial variability of (a) IL-1β, (b) IL-6, (c) tumor necrosis factor alpha (TNF-α), and (d) matrix metalloproteinase 1 (MMP-1) mRNA expression. Three biopsies each from five rheumatoid arthritis (RA) synovial tissues were analyzed by quantitative PCR using the cellular standard curve method. Results are expressed in relative expression units (REU). Data are log-transformed and the mean ± standard deviation is indicated.