Decreased binding of nuclear extracts from aged macrophages to the W and X boxes of the IAβ promoter. Nuclear extracts (4 μg) were used for gel retardation assays with probes for W, X, and Y boxes. A 50-fold excess of cold DNA was used in the competition experiments. The amount of DNA free or bound to protein was quantitated, and the figure represents the mean ± SD of five independent experiments. There are significant differences between young and aged macrophages in the binding capacity to the W and X boxes (P < 0.01), but not to the Y box (P > 0.05).