text: Dialkylating agents can react with two different 7-N-guanine residues, and, if these are in different strands of DNA, the result is cross-linkage of the DNA strands, which prevents uncoiling of the DNA double helix.  If the two guanine residues are in the same strand, the result is called limpet attachment of the drug molecule to the DNA. Busulfan is an example of a dialkylating agent: it is the methanesulfonate diester of 1,4-butanediol. Methanesulfonate can be eliminated as a leaving group. Both ends of the molecule can be attacked by DNA bases, producing a butylene crosslink between two different bases.
  Monoalkylating agents can react only with one 7-N of guanine.
  Limpet attachment and monoalkylation do not prevent the separation of the two DNA strands of the double helix but do prevent vital DNA-processing enzymes from accessing the DNA.  The final result is inhibition of cell growth or stimulation of apoptosis, cell suicide.
  Examples
  In the Anatomical Therapeutic Chemical Classification System, alkylating agents are classified under L01A.
  Classical alkylating agents
  Many of the agents are known as "classical alkylating agents". These include true alkyl groups, and have been known for a longer time than some of the other alkylating agents. Examples include melphalan and chlorambucil.
  The following three groups are almost always considered "classical".
  Nitrogen mustards
  Cyclophosphamide — the most widely used alkylating agent of modern times
  Chlormethine also known as mechlorethamine or mustine (HN2) — the first alkylating agent to receive regulatory approval
  Uramustine or uracil mustard
  Melphalan
  Chlorambucil
  Ifosfamide
  Bendamustine
  Nitrosoureas
  Carmustine
  Lomustine
  Streptozocin
  Alkyl sulfonates
  Busulfan