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Histone methyltransferases catalyze site-specific deposition of methyl groups, enabling recruitment of transcriptional regulators. In mammals, trimethylation of lysine 4 in histone H3, a modification localized at the transcription start sites of active genes, is catalyzed by six enzymes (SET1a and SET1b, MLL1–MLL4) whose specific functions are largely unknown. By using a genomic approach, we found that in macrophages, MLL4 (also known as Wbp7) was required for the expression of Pigp, an essential component of the GPI-GlcNAc transferase, the enzyme catalyzing the first step of glycosylphosphatidylinositol (GPI) anchor synthesis. Impaired Pigp expression in Wbp7^−/− macrophages abolished GPI anchor-dependent loading of proteins on the cell membrane. Consistently, loss of GPI-anchored CD14, the coreceptor for lipopolysaccharide (LPS) and other bacterial molecules, markedly attenuated LPS-triggered …