Immune cells from some patients can recognize spreading tumors and attack them. Steven Rosenberg of the National Cancer Institute and his colleagues cloned the genes governing the cancerrecognizing receptor in these immune cells—called tumor-infiltrating lymphocytes— from a patient who had successfully beaten back melanoma. They introduced this genetic information via retrovirus into regular T cells from 17 melanoma patients. After chemotherapy, the scientists infused the engineered lymphocytes back into their patients and discovered that such cells could persist, making up between 9 and 56 percent of the T cell population one month after treatment in 15 of the 17 patients. Even better, the cancer disappeared in two patients, who remain disease-free 18 months after treatment and continue to exhibit high levels of the engineered immune cells in their blood. The work was published online August 31 by Science.