This leads to the transcription of genes that promote cell division. Cancer is the loss of cell cycle control: cells divide continuously and uncontrollably, rapid growth and division results in a mass of cells that can interfere with body functions. Chemotherapy targets cell cycle proteins to stop cell division: taxol freezes the mitotic spindle by preventing de-polymerization of microtubules. Two general types of defects: 1. Defects in proteins required for cell growth and division are always active. Ras is a component of a signal transduction cascade that triggers cell growth. Defective forms of ras in cancerous cells can"t be inactivated: defects in tumor suppressor proteins mean cell cycle can"t be stopped. P53 is a tumor suppressor that prevents cells from dividing if dna is damaged. In many cancers, defective p53 allows cells with defective dna to proceed through cell cycle instead of arresting cell cycle until damage fixed, or triggering cell death.