Scientists Discover Molecular Switch (IMAGE) University of Veterinary Medicine -- Vienna Caption Dagmar Gotthardt and her colleagues have shown for the first time ever that NK cells produce a factor called VEGF-A that promotes tumor growth. Credit (Photo: Petra Kudweis) Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.