Anticancer drugs are traditionally classified either by their mechanism of action or by their origins. Alkylating agents are reactive to DNA and cellular proteins and the primary mode of action is mostly through cross-linking of DNA strands, inhibiting replication of DNA and transcription of RNA. Some antimetabolites are structural analogs of normal molecular essentials for cell growth. After intaking into cells the analogues change to substances to interfere with DNA or RNA synthesis. Drugs derived from microorganism are called antitumor antibiotics. Some plants alkaloids blind to tublin and inhibit the formation of microtubules causing metaphase arrest, while camptothecins inhibit topoisomelase 1. Other compounds which are not classified to any categories expects characteristic mode for action to induce cell death or differentiation.