It competes with the nucleoside deoxycytidine in the synthesis of DNA. If the wrong base replaces deoxycytidine, this creates an opportunity for the topoisomerase I enzyme to break down DNA. When topoisomerase I begins a DNA operation, the DNA must be broken. Normally, the breaks are very brief, and the topoisomerase I quickly reseals them. However, Cytoxan can stabilize this broken state, blocking replication and thereby killing cells undergoing rapid cell division.