There are two important ways in which studies of rare and common variants might intersect. The first is the possibility that common variants may act as significant modifiers of the effects of rare variants (see ref. 31 for an example). This could be investigated, for example, by looking at the effects of established common variants influencing breast cancer susceptibility on the ORs for putative rare variants at the BRCA1 and BRCA2 loci (Box 2). The second point of interaction is that the genes for which common variants are found, or genes nearby that may contain the functionally relevant variant, could be considered candidates for the search for rare variants. They may also then help identify the functional variant associated with a common disease variant.
