In all three of our examples, something went wrong--the DNA lead to a mutated RNA copy and formation of new mutated or cancerous cells arose. Cancer cells divide at a much quicker rate than normal cells, forming irregularly sized cells with enlarged nuclei. As cancer cells continue to divide, they form masses called tumors. Tumors can sometimes be treated with new molecular agents such as the breast cancer drug Herceptin. This drug can be injected into a patient to try and repair the DNA copying process that got out of control, destroy the tumor so that only normal cells remain and future cell division only originates from normal, unmutated DNA.