DNA methylation analysis. (A) Filter hybridization analysis of Fau I cleavage of Hae III-digested genomic DNA prepared from transgenic splenocytes and T lymphoblasts. Fau I recognises the Sp1 site in the human granulocyte-macrophage colony-stimulating factor (GM-CSF) promoter and is located within a 175 bp Hae III fragment which was used as the probe. DNA was purified from either whole spleen or from cultured T cells previously activated with ConA (T blasts). 5 μg of each DNA sample was digested with either just Hae III, or Hae III plus Fau I. After electrophoresis, DNA was electrophoretically transferred from a polyacrylamide gel to a nylon membrane and hybridized with 32P-labelled probes. (B) Reprobing of the same membrane used in (A) with a DNA fragment encompassing a CG island within the mouse adenine phosphoribosyltransferase gene.