Commentary

There are two biological properties of genomic methylation patterns that can be regarded as established. First, methylation of 5′-CpG-3′ dinucleotides within promoters represses transcription, often to undetectable levels. Second, in most cases methylation patterns are subject to clonal inheritance. These properties suit methylation patterns for a number of biological roles, although none of the current hypotheses can be regarded as proved or disproved. One hypothesis suggests that the activity of parasitic sequence elements is repressed by selective methylation. Features of invasive sequences that might allow their identification and inactivation are discussed in terms of the genome defense hypothesis. Identification of the cues that direct de novo methylation may reveal the biological role (or roles) of genomic methylation patterns.

On 3 October 1995, O.J. Simpson was acquitted of two murders in spite of very strong DNA evidence linking his blood to the crime. Although numerical statements describing the strength of this evidence were made, the DNA profiles included so many loci that the need for presenting numbers in this case, and in others using similarly high numbers of loci, is probably unnecessary. If numbers are to be presented, however, they should be given in the form of likelihood ratios. One thing the verdict in the Simpson case makes clear is that it is essential that the integrity of DNA evidence (with regard to collection, potential contamination or tampering) be beyond doubt.