The CpG islands found at the 5′ ends of many mammalian genes are typically
unmethylated despite being both exposed to diffusible protein factors in nuclei and rich in
CpG, the target site for DNA methyltransferase. We show here that the CpG islands
associated with the human Thy-1 and profilin genes are inherently resistant to de novo
methylation by purified murine DNA methyltransferase, and that the higher than expected
tendency of CpG sites in islands to be flanked on both sides by GC base pairs is the likely …