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The 'histone-code' hypothesis proposes that cell fate 'decisions' are achieved through the creation of stable epigenetic histone 'marks' at gene loci. Here we explored the formation of marks of repressive methylation of histone 3 at lysine 9 (H3-K9) and of H3-K27 at the locus encoding interferon-gamma (Ifng locus) during the commitment of naive T cells to the T helper type 1 (TH1) and TH2 lineages. Methylation of H3-K9 across the Ifng locus was rapidly induced in differentiating TH1 and TH2 cells and was sustained in TH1 cells. In contrast, TH2 differentiation caused loss of methylation of H3-K9 and gain of methylation of H3-K27 by mechanisms dependent on the transcription factors GATA-3 and STAT6. Thus, histone-methylation marks at the Ifng locus are highly dynamic, which may ensure higher-order transcriptional regulation during early lineage commitment.