ANIMAL somatic cell DNA is characterized by a bimodal pattern of methylation: tissue-specific genes are methylated in most cell types whereas housekeeping genes have 5′ CpG islands which are constitutively unmethylated^1,2. Because methyl moieties derived from the gametes are erased in the morula and early blastula³, this profile must be re-established in every generation; this is apparently accomplished by a wave of non-CpG island de novo methylation that occurs at implantation⁴. Using transfection into embryonic stem cells and transgenic mice as a model system, we now show that Spl elements play a key role in protecting a CpG island in the adenine phosphor!bosyItransferase (APRT) gene from de novo methylation. This recognition mechanism represents a critical step in embryogenesis, as it is responsible for setting up the correct genome methylation pattern which, in turn, is involved in regulating basal gene expression in the organism5.