The unit is defined as protection from cleavage by the BstUI restriction enzyme (CGCG). BstUI is blocked by hemimethylation within its recognition site. SssI will completely methylate the BstUI site. This was tested using H3 incorporation into oligonucleotides containing all possible combinations of methylated, hemimethylated and unmethylated BstUI sites. SssI methylated all sites well (D. Robinson unpublished observation). For complete methylation 4 units per μg of DNA is recommended.