MMS methylates DNA predominantly on N7-deoxyguanosine and N3-deoxyadenosine, and to a much lesser extent also methylates at other oxygen and nitrogen atoms in DNA bases, and also methylates one of the non-carbon bound oxygen atoms of the phosphodiester linkage. Originally, this action was believed to directly cause double-stranded DNA breaks, because homologous recombination-deficient cells are particularly vulnerable to the effects of MMS. However, it is now believed that MMS stalls replication forks, and cells that are homologous recombination-deficient have difficulty repairing the damaged replication forks.