A class of cytotoxic drugs that prevent cell replication by binding to DNA and preventing the separation of two DNA chains during cell division. They are widely used to treat a variety of cancers. These drugs cause the side effects common to all cytotoxic drugs but are particularly damaging to eggs and sperm and may therefore cause infertility. Their long-term use has also been associated with an increase in the incidence of non-lymphocytic leukaemia. The common alkylating drugs are cyclophosphamide, ifosfamide, chlorambucil, melphalan, busulfan, lomustine, carmustine, estramustine, treosulfan, and thiotepa.