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During their development, T lymphocytes make sequential cell fate choices: T rather than B lymphocytes, then TCRαβ or TCRγδ, CD4 or CD8, and Th1 or Th2 lineage. These fate choices require the initiation of new programs of gene expression, and once initiated, these programs must be faithfully propagated in a heritable manner from parental cells to their progeny. With the exception of the T cell receptor, these changes in gene expression occur without a change in information encoded directly in the DNA sequence. Rather, these heritable programs of gene expression are imposed, at least in part, epigenetically through changes in chromatin structure and DNA methylation, allowing T cells to tune the threshold for expression of specific genes.